What began as a normal Sunday night set on Caxton Street for one Brisbane band turned into an impromptu jam session with Red Hot Chili Peppers' Chad Smith.

The American drummer, currently touring Australia with the world-famous rock band, walked into Lefty's Old Time Music Hall just as Golden Age of Ballooning took to the stage.

Guitarist Jack Bratt said Smith "sat at a table right next to the stage and watched the whole set".

"After the set I went over to say hello after, thanked him for coming to the show and how it's an honour to meet him," Mr Bratt said.

"One of the guys sitting with him said we should get Chad to jam with us … I thought, 'that's ridiculous, he'd never want to do that' and Chad said 'I'd love to, but I don't want it to be too much trouble.

Red Hot Chili Peppers' drummer Chad Smith (centre) after a jam session with Harry Hurst (left) and Jack Bratt (right). (Facebook: Golden Age of Ballooning)

"So I got our bass player Harry [Hurst] and I, we set our gear back up and he got up and we played. It was unbelievable.

"We did a Chili Peppers-style jam with him. There was probably 50 people there but literally everyone had their phone out filming it.

"We just did the one jam, it was a bit of a blur but I say it was between five to 10 minutes … it was an improvised jam."

For Mr Bratt, a Chili Peppers fan for decades, the encounter was beyond anything he had ever imagined.

"You can't even make it up. I literally have not dreamed of that scenario, and it happened. It's just so out of this world."

The Chili Peppers die-hard flew to Sydney to watch the band perform in Sydney last week, and has tickets to Monday's show at Brisbane Entertainment Centre in Boondall.

Smith also put him on the guest list for Tuesday's show.

"The Chili Peppers were the first band that turned me onto music when I was 12. He [Smith] feels like a person I know even though I've never met him before," Mr Bratt said.

"I was probably a lot calmer than I should've been … it felt quite normal and comfortable to chat to him as an equal as opposed to a fan. He was super nice, humble, loves music and having a beer, and so do I."