There are only a few shots in Tamra Davis’ documentary The Best Summer that were filmed this century. They’re the very first ones: shaky footage of a clattering box of videotapes being loaded hurriedly into a car boot as Davis evacuated her home in Malibu during the Palisades fires last year. Almost everything else in the film was in that box, forgotten for nearly three decades, a time capsule of a one-off tour of Australia and Asia featuring some of the most influential musicians of the 80s and 90s.

Summersault, pulled together by twentysomething promoter and future Modular Records founder Steve “Pav” Pavlovic, brought Sonic Youth, Pavement, the Beastie Boys, Bikini Kill, Beck and more to five Australian cities over new year’s 1995/96. It had started as Pav booking a Beasties tour that snowballed as the bands – many of them close friends – liked the sound of a break down under.

At the time, Davis was newly married to Michael Diamond, better known as Mike D of the Beastie Boys. (She’s “Tammy D gettin’ biz on the crop” on Intergalactic. They’re now separated, but still close – Diamond was one of the first people to see this film.) She was also fresh from directing Billy Madison and had dozens of music videos under her belt – including Sonic Youth’s MTV classic Bull in the Heather, starring Bikini Kill’s Kathleen Hanna. “I was always filming” at Beasties and Sonic Youth shows, says Davis, which meant she could be both a friend and a fly on the wall backstage.

She and Hanna paired up early and semi-seriously decided they were making a documentary – “you know when you find your best friend at summer camp and come up with a project to do together?” – with Davis shooting live sets, and Hanna interviewing their peers in green rooms and behind portable toilets.

The footage is raw and the live sound is rawer. Davis did waver on whether she should take the opportunity to tidy them up; Pavlovic offered to track down cleaner audio he knew to be somewhere in Australia’s national youth radio station Triple J’s archives. “And then, in the end, all the bands were like, ‘No, we like this’, they liked the sound.”

Her sons, Skyler and Davis Diamond – now in their early 20s, with their own project Very Nice Person – also convinced her not to polish too much. “I showed them early cuts and they liked when my camera would go out of focus. They were like, ‘Keep that in, Mom, that shaky camera … That makes us feel like it’s real’.”

Hanna is the film’s effervescent anchor, and Davis’ gaze becomes the audience’s POV. “I really wanted you to feel what it’s like to be on a festival tour and backstage,” she says. “At no point does it cut to another person’s point of view. And I also didn’t want to cut to everybody now.”

After Davis’ “icebreaker” questions – what are you reading? What’s your favourite colour? – Hanna hits them with big ones: what is being on stage like for you? And how do you manage the boundary between public and private personas? Watching these indie-rock stalwarts, some already veterans and some still green, grapple with this question from a peer is perhaps the most intimate part of the film – from Hanna being visibly discombobulated by a flirty Beastie (Adam “Ad-Rock” Horovitz, who she later married) to a startlingly willowy, doe-eyed Dave Grohl describing the terror of being behind a mic instead of a drum kit.

Davis didn’t know Grohl well at the time, even after her years in the Nirvana/Sonic Youth circle – “[Backstage] I would always talk to Kurt [Cobain] because Kurt was obsessed with movies and film, so he would always try to ask me a million questions!” – and she was struck by the starkness of the newly minted frontman’s growth over those short weeks. “I just see this vulnerability of somebody that was always in the background, the drummer guy that you don’t really talk to so much,” she says. “And you see that he finds his voice by the end of this tour. Seeing that progression was just so beautiful to me. He just gets through it.”

Eschewing narrative crutches like voiceover or talking heads preserves the time-capsule bubble in both directions, even as the audience knows the grief lurking out of frame. Cobain’s death in 1994 was still fresh in the rearview, but the subsequent 30 years have often been cruel too; Grohl, most notably, would lose another bandmate, more friends. The Beastie Boys came to an end in 2012 when Adam “MCA” Yauch died of cancer aged 47.

“I kind of was making a film that showed the loss of our youth,” Davis says, matter-of-factly. “Adam is so alive in that footage, it’s just so beautiful, and there’s nothing hinting that one day he’s not going to be there. And the same with seeing Jimmy [Swanson, Foo Fighters roadie who died in 2008], Dave’s best friend, he’s no longer with us … There was this sense of what has happened before, but also the loss that happens after.”

Davis also sees the advent of the internet and smartphones as “a loss of innocence” for live music in particular: “There’s no phones – when the audience reaches out, there’s a direct connection. And in a year or two, all that’s going to be lost.”

The Best Summer isn’t the first time Davis has mined her own archive. Working at Francis Ford Coppola’s production company in the early 80s and then an LA gallery, she befriended Jean-Michel Basquiat, and filmed a rare, intimate interview. When the artist overdosed in 1988, a grieving Davis initially stashed her footage away, but later turned it into a short film, then a feature-length documentary. After a screening, Davis says she was approached by women who “were so struck by how he looked at me in the camera and how he spoke to me, knowing that that’s not how he would ever talk to a male interviewer … [They] were like, ‘it’s the first time I really noticed how a girl behind the camera can really have an impact on who they’re speaking to’.”

One of Davis’ guiding principles is to inspire young women in film and she’s refreshingly proud of her younger self’s raw camera work. “Most of those songs are one take, one operating take,” she grins. “I’ve done 150 music videos and I operated a lot myself. So when I watch it back now, I look at it and go, ‘Wow’. My camera skills were always dope.”

Watching The Best Summer as an Australian of a certain age, the nostalgia is searing. Not just for 90s festivals, but for the whited-out glare of high summer, an extended family squinting on sun loungers and chatting on battered couches, just like home camcorder footage of baking hot Christmas Days.

“That’s the biggest criticism I got on this, like, ‘oh, we’re watching Tamra Davis’ home movies’,” the director laughs. “Yeah, maybe you are! And what a great home movie I had … In a way, I was trying to make a film that also inspires just one girl to make a movie. Maybe you could make a movie about your cool group of friends and tell a story and show us your world.”

• The Best Summer screens at the Melbourne international film festival on 7, 8 and 14 August and will be in Australian cinemas later this year