Thousands of people have poured into Sydney's ANZ stadium to show support for the state's firefighters and all those impacted by the summer's bushfire crisis.

More than 70,000 people are expected at the Fire Fight Australia concert on Sunday, which according to organisers sold-out within 24-hours of tickets going on sale.

Hosted by comedian Celeste Barber, who has raised more than $50 million for bushfire relief, all ticket profits will go to groups which are helping fire-ravaged communities get back on their feet.

Andrea Gartside decided to attend the nine-hour concert with her friend Maureen Mackie to support the firefighters after the unprecedented fire season.

"We're doing our part in helping," she told AAP at ANZ stadium.

"It's going to be awesome ... it's great that it's bringing everyone together for a good cause."

Rock behemoths Queen + Adam Lambert will headline, with Alice Cooper, Irish favourite Ronan Keating and singer-songwriter k.d. lang also performing.

Queen fan Cheryl Hind says she's excited to see the band perform again.

"It's part of why I'm here today and to thank the firies for everything they've done," she told AAP.

Lisa Yasso travelled from central Queensland to see the legendary rock band perform again.

"It's an awesome cause," she told AAP.

Canadian crooner Michael Buble will also feature as he'll be beamed in from his concert in Melbourne.

The international stars will be joined by popular Australian acts spanning generations, including 5 Seconds of Summer, Amy Shark, Daryl Braithwaite, Delta Goodrem, Grinspoon, John Farnham, Lee Kernaghan, Olivia Newton-John and more.

The concert kicked off at 1pm AEDT and will be broadcast live on Channel 7 and across the world on Foxtel, with a free public screening in Melbourne's Federation Square.

Fire Fight's profits will be shared between the Red Cross' disaster relief and recovery fund, rural and regional fire services and the RSPCA's bushfire appeal.

Donations to the Foundation for Rural and Regional Renewal will also be accepted during the concert, to fund grants for community-led projects needing support in the bushfires' aftermath.