A signed dress worn by Serena Williams at her first tournament of the year is being auctioned off to raise funds for firefighters tackling Australia's unprecedented bushfire emergency.

Williams, who is preparing for this month's Australian Open at the WTA Auckland Classic, joins a slate of sports stars from around the world who have stumped up memorabilia and cash to help pay for firefighting efforts.

It came a day after reigning Formula One champion Lewis Hamilton pledged $500,000 for relief efforts, saying that he had been deeply saddened by reports that over a billion animals had been killed in the fires.

Earlier on Friday, Australian cricket legend Shane Warne raised more than Aus$1 million for bushfire victims by auctioning off the "baggy green" cap he wore through his Test career.