In Denver last year, an overeager fan chucked a gigantic bra onstage at Australian singer/songwriter Missy Higgins. Nonplussed, Higgins picked it up, clasped the monstrous thing on over her shirt, and proceeded to execute a tongue-in-cheek rendition of her ballad "Don't Ever." Picture no-frills Higgins — who tends toward loose-fitting, muted colors and never, ever looks made-up — crooning sweetly "we'll get a house where the trees hang low, and pretty little flowers on our windowsill will grow; we'll make friends with the milkman, and the butcher Mr. Tims will give us discounts when he can." And all the while, she was pinging out saccharine notes on a waist-high xylophone and swaying with her hands over her heart like the angelic '50s housewife she's clearly not.

"I think they overestimated me just a tad," Higgins joked, looking down at the enormous white number bagging out over her small chest.

It's how she rolls, ladies and gentlemen. Even for an Australian, Missy Higgins takes the cake on unpretentiousness and goofy modesty. Earlier this month, I crammed into Terminal 5 — Manhattan bastion of indie rock — to watch the earnest, bed-headed, self-effacing 25-year-old wrap her headlining U.S. tour behind On A Clear Night, her second album. Although her quietly catchy ballad "Where I Stood" has hit the soundtracks of major TV shows like Grey's Anatomy and The Hills, and although she's enjoyed platinum status back home in Australia since the 2005 release of her first album, Sound Of White, everything about her demeanor onstage and off seems to refuse stardom.

Maybe it's the total unaffectedness of her rich, clean, sometimes husky voice, or the way she says "thank you so much" after each song and seems to really mean it (her first opener, Justin Nozuka, closed his set with the requisite "thank you, I love you," but I'm pretty sure what he meant was "thank you, I love me, too"). Maybe it's the simple white (un-ironed) oxford shirt, vest, plain black skirt (just short enough to hint at what a mean rock-climbing habit will do for your figure), and slouchy ankle-high boots. Or the slightly nerdy way she stomps her feet during feistier tunes like "100 Round the Bends," a pulse-quickening roller coaster of a song about "being addicted to dysfunctional relationships."

Or maybe it was the uncanny way she surrendered to "Forgive Me," a heartbreaking, melancholy plea for mercy from an unfaithful man to his wife, son and God, for, as Higgins succinctly puts it, "being a bit of a bast*rd, basically." Watching Higgins' eyes squint as she tapped into the man's anguish — "Oh, my son, look at what I've done / But I am learning still, I am learning still, know that I am learning still / And oh, my wife, you are my life, and I am burning still, burning still, know that I am burning still / And all of my light is for you, and home's anywhere you are too" — I absolutely felt the narrator's remorse in Higgins' unfussy minor chords. Her selfless capacity for tapping into a fictional stranger's pathos is almost eerie — certainly beyond her 25 years.

Maybe it's the fact that Higgins almost seems to enjoy herself more when she's screwing up royally than when things are going according to plan. Case in point: Higgins started her Terminal 5 set on a slightly deflated note, settling wordlessly at the keyboard and delivering "They Weren't There" as though she's wasn't quite there. It wasn't until a couple songs later, when she jumped into "The River" — a smooth, jazzy, Norah Jones-esque tune about a 10-year-old girl who drowns herself after being abused at home — in the entirely wrong key, that she lit up.

"Oh ... sorry, I was in the wrong, ah, key," she chirped, grinning impishly. "Let's have a disco dream sequence to get out of that." Scaling her way up the ivories, Higgins narrated in a horror-movie voice, "It was all a bad dream. Let's start again, shall we?" Looking up as though she'd only just noticed the hundreds of us spread out before her, packed in like adoring sardines, she gave a hand-pumping "whoop whoop!" and began to soar through the rest of her set at full throttle.

Higgins' modesty, disinterest in rock stardom and penchant for the humanizing blooper are what make her exactly the kind of environmentalist you'd want to be friends with. She's all practice and no preach. A vegetarian, Higgins gave out her hit single "Where I Stood" for free to benefit the Sierra Club's Two Percent project, loves rock climbing in Joshua Tree and camping in the Sierra Nevada (she's been LA-based for the past couple of years), partners with Cliff Bar's Green Notes program to green her tours and educate fans, and webcasts eco-education video blogs from her carbon-neutral tours. Have I mentioned that Higgins brought the house down at Live Earth Australia, the Aussie branch of that little multicontinent bonanza Al Gore threw for Mama Earth in 2008?

No surprise that Billboard magazine ranked Higgins among the top 10 greenest musicians on the scene last year, right alongside the likes of Jack Johnson, the Roots and Willie Nelson. But Higgins would never admit to her green elite status. "For me, it's more of a personal thing," she insisted in an interview a week after her NYC show. "I don't judge any musician or nonmusician for living the sort of lifestyle that they do. I think it all comes down to conscience and making the decision for you and your life. Everyone's at a different point of realization."

In keeping with this no-judgment, no-preach policy, Higgins tends to leave her politics offstage these days. Put in her place, I'd be all "hey, before I play that hit single you're itching to hear, can I tell you how much water you waste every time you pre-rinse your dishes?" Instead, Higgins simply sets a good example, taking a swig now and then from her reusable SIGG bottle (it's green — no, literally), or mentioning offhandedly that you might want to check out the Oxfam booth on your way out. "The first few tours I did, I was talking about [green issues] on stage," Higgins told me over the phone. "Then I just figured I probably shouldn't talk about it every tour, so now I guess I'm just talking about it in interviews. It's something I've been doing for quite a while now, so it doesn't feel like news."

Don't go digging for global warming or deforestation in Higgins' lyrics, either. "I just tend to write songs about more cerebral things, more just emotions, abstract or personal," she says. "Whenever I try to write about something literal or more political or environmental, it just doesn't sound right coming out of my mouth. That's why I've just got to talk about it and take action instead." Personally, I'm relieved to know that Higgins isn't going to try to jam trash gyres or the importance of composting into her next album. No one likes to see art sacrificed to politics, and let's face it, even Jack Johnson — crown king of surfer dude eco-tunes — loses a bit of his mojo when he gets too preachy in his lyrics.

But while it doesn't look like Higgins will sing out against the oil companies anytime soon, it should be said that her most triumphant and euphoric songs are the ones inspired by the natural world. "Steer," a take-me-higher single about breaking free from society's petty demands (put it on your jogging playlist, between "In The Name of Love" and "Chariots of Fire"), came to Higgins with a simple glance up at the clear night sky in the rural Australian northwest. Having grown up in Melbourne, Higgins had moved up to Broome, to write her second album in the still, open space.

"It was the first place I'd ever felt honestly connected with my country, with the physical land of my country," Higgins says. "It's just so ruggedly beautiful, Broome is. You can feel the heat of the red Pindan soil under your feet, and the sky at night is the clearest night sky I've ever seen. You really feel like you're sitting underneath endlessness. This night I was just looking up and I felt so tiny amongst it all, I felt so insignificant. That really empowered me, because I realized how short my life was in the grand scheme of things. I think it really took getting out of the city for a while and literally putting my feet on nature, getting reconnected with my country to remember who I was and get my priorities straight."

Likewise, in "Going North," an almost-country song about her move to Broome, Higgins radiates with the calm elation that comes from stepping off the map and into the wild. Belting the song out in Terminal 5 — "I want to dance, the tango with chance, and I want to ride on the wire / Cause nothing gets done with dust in your gun, and nobody respects a liar / So goodbye for awhile, I'm off to explore / Every boundary and every door / Yeah I'm going north" — Higgins is as exuberantly freewheeling as John Denver on his country roads. I can practically feel the hot red Pindan soil under my feet.

Ironically, Higgins would love to see Australia — which she says sees itself as a country of "farmers and miners and laborers" — lose the very "no worries" attitude and unassuming modesty that make her such an inspiring performer. "I think as a country we need to stop relying on what we can dig up from the ground and what we can suck out of the Earth," she tells me. "We've got some really great brains in our country, and some really great potential to be at the forefront of environmental science research, but we see ourselves as a working-class kind of economy."

First on her sustainability agenda for the Land Down Under? Solar power on a large scale, green jobs and better water-management education for drought-stricken farmers who are being forced to watch helplessly as their land turns to dust. Lofty goals, but if her do-something-about-it track record is any indication, Higgins will be donating new hit singles to benefit drought-management education before you can say triple platinum.