Volunteer firefighters are scorching Australia's forest undergrowth, reducing fuel for what is expected to be the fiercest fire season since the monster "Black Summer" blazes.

Deadly wildfires have devastated forests in Canada, Greece, Hawaii and elsewhere around the world this year but unlike many other countries, Australia heavily relies on a 190,000-strong volunteer cohort to battle the flames.

Their courage was on display during the "Black Summer" fires of 2019-2020 that killed 33 people and millions of animals, as well as destroying thousands of homes and razing vast swathes of eucalyptus forest.

But some of them fear their brigades may not be able to cope in the future should global warming make fires even more intense and frequent.

"It's terrifying but, if 2019-20 became the norm, I don't know how you sustain that year on year. I don't think that's sustainable," said Andy Hain, a 41-year-old volunteer with the NSW Rural Fire Service, who is married with two young sons.

Faced with repeated fires on the scale of the "Black Summer", Australia and other countries would have to share personnel and resources more than they do already, said Hain, who has volunteered for nearly 10 years in Picton, a rural town southwest of Sydney with a population of about 5,000.

Because of wet conditions since "Black Summer", the RFS warns that the threat of grass fires is the highest it has been in two decades