We first meet Barnett in the artist village at Pukkelpop, a Belgian festival near the sleepy town of Hasselt. She shows us round her white cabin. “This is our little base,” she laughs. “It’s not as depressing as it looks. We don’t normally spend time sitting in here, we’re normally outside, as festival sites are so beautiful to sit around in in the sun.”

“They don’t look too comfy,” says Sloane, “but you get to the point that you can find a spot to sleep. Under the table…”

Barnett, Sloane and Mudie have certainly had the practice. In the past two years, Barnett has become better known around the world than most of the Australian musicians who inspired her songwriting, such as Darren Hanlon, The Triffids’ David McComb, You Am I’s Tim Rogers and Paul Kelly. Highlights such as “Avant Gardener” and “History Eraser” (from “A Sea Of Split Peas”) are catchy, clever and lightly psychedelic. Her debut album proper, this year’s excellent Sometimes I Sit And Think, And Sometimes I Just Sit, meanwhile, introduced a punchier, more dynamic sound and expanded her songwriting into caustic grunge (“Pedestrian At Best”), taut short stories (“Elevator Operator”) and Crazy Horse-esque epics (“Kim’s Caravan”). If anything, Barnett’s now revered even more. They’re shaping up for their biggest ever UK tour, including two nights at London’s Forum.

“I think Courtney strikes so many people because she’s a very relatable person,” says Sloane, pondering why Barnett has been so well-received. “It’s nice to have lyrics like that as well. Most of the music you hear, some of it’s really good, but sometimes the lyrics are just like, y’know… ‘Take your fucking shirt off, I wanna dance!’”

“Her voice is unique,” adds Mudie. “I’ve never heard anyone sing like it. She also shreds the fuck out the guitar!”

Barnett herself is unsure why she’s connected so widely playing such nuanced, personal music, and seems almost embarrassed about her success. “It’s a hard thing to explain, because I’m still trying to figure it out. I’m constantly trying to make sense of the music industry, trying to understand things, but I don’t think you really can. I think you’ve just got to go with it and do what feels right, without compromising your morals. Maybe something I’m saying is different somehow.”

As well as being taken on tour by Blur, Barnett was invited by Jack White to record a single for Third Man’s Blue Series when the trio were in Nashville earlier this year. Just released, it pairs Barnett’s “Boxing Day Blues Revisited” with a lonesome cover of “Shivers”, originally written by Rowland S Howard and sung by Nick Cave when they were both members of their proto-Birthday Party group The Boys Next Door.

“It was just the craziest day ever, recording that,” says Mudie. “It was like meeting one of your heroes. Jack White’s a pretty kooky dude, and it’s a pretty kooky house. It’s a bit like Charlie And The Chocolate Factory in there. He’s got a bowling alley in his lounge room.”

Though there’s no bowling alley in their festival cabin at the sweltering Pukkelpop, Barnett entertains herself by continually strumming and picking on her new acquisition, a tiny Taylor Baby guitar she bought in LA. “It’s good for warming up before we play,” she explains, “’cos I fucked my throat up real bad last year, from singing and not doing vocal exercises. I’ve just been writing all these new songs and mucking around with folk songs.

“I’ve got nearly an album’s worth of country/folk/picky songs that just haven’t fit anywhere else,” she reveals, “which are kind of coming back out. So those are fun, messing around with that shit. I forget songs so easily sometimes. I’m a big planner, but I’m bad at following through. I’m sure slowly I’ll do an album, or something, with them. Sometimes it’s hard, songs just don’t feel like they fit together with others. But I’m sure they’ll come out in the end. I don’t know what the next album will sound like, anyway.”

Just before stage time, the trio change their clothes (Barnett dons a Modern Lovers T-shirt), and all jump in a people carrier to be driven round to the back of the large tent they’re set to perform in. The space is packed by the time they’re introduced in Flemish by a man and his young daughter, and duly rip into “Elevator Operator”. Even compared with the louder Sometimes…, the band are powerful and raw here, with Barnett’s playing gloriously ragged on the turbo-charged “Lance Jr”, originally from her hushed debut EP, 2011’s “I’ve Got A Friend Called Emily Ferris”, and Sometimes…’ Pavement-like waltz, “Small Poppies”. There used to be a second guitarist in the group, Dan Luscombe, but since he left last year to devote more time to The Drones, Barnett has taken over all guitar duties – much to the benefit of their performances, as she’s developed into a fascinating, primal guitarist. “They’ve all grown so much, those songs,” says Barnett afterwards. “I think it’s just from touring, songs grow with you and with the environment that you’re in. I’ve grown a lot more as a guitarist, too, and when I was writing those early songs I was in my bedroom, so they came out quite timid-sounding. Being forced to play as a three-piece, I freaked out. It was like, ‘Oh my God, we can’t do it,’ because I wrote them as these full songs with extra melody lines, but I reckon that forced me to think and play differently, which is good.”

Just as she switched from a four-piece to this trio, though, the guitarist is certain that things will change in the future. “I’m sure throughout my life I’ll do lots of different varieties of bands, do one of those big orchestral albums and a solo album on piano and all that shit,” she says, tongue in cheek but also deadly serious. “Can’t do the fucking same thing all the time, it would be so boring. But yeah, I love [the trio] for now. It feels like the perfect release of energy. I’m sure the band will grow and shrink over years of doing different things, like our electronic LP and [laughs] all that stuff. But I’m sure the core will stay the same.”