(CNN) Australian Prime Minister Scott Morrison said on Friday that he would return from his holiday immediately, after two volunteer firefighters died battling wildfires on Thursday.

Morrison has been heavily criticized for taking a pre-Christmas break in Hawaii, as more than 2,500 firefighters across New South Wales state work to contain raging fires . He said Friday that he would "happily" return to deal with the crisis.

He then added, "I don't hold a hose, mate, and I don't sit in a control room. That's the brave people who do that are doing that job. But I know that Australians would want me back at this time."

His return comes after volunteer firefighters Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, died Thursday near the town of Buxton, southwest of Sydney. It's believed their vehicle hit a tree before rolling off the road, said the New South Wales Rural Fire Service in a statement on Friday . Three other firefighters were also injured.

Keaton joined the RFS as a volunteer firefighter in 2006 and was deputy captain of the Horsley Park Rural Fire Brigade in western Sydney. O'Dwyer joined the service in 2003 and was also a member of the Horsley Park brigade. Both men were fathers to young children, according to CNN affiliate Nine News.

"This is an absolutely devastating event in what has already been an incredibly difficult day and fire season," said the RFS statement.

The firefighters' deaths were met with an outpouring of grief across the country on Friday. Fellow volunteers and members of the public left flowers, cards, and other tributes at Horsley Park fire station. NSW Premier Gladys Berejiklian ordered flags to be flown at half-mast statewide on Friday, Nine News reported.

"NSW has tragically lost two heroes," Berejiklian tweeted on Friday. "Our thoughts are with their loved ones and the extended RFS family as we come to terms with the devastating news."

On Friday, Morrison expressed condolences for Keaton and O'Dwyer's families, calling them "amongst our most courageous Australians."

He also expressed regret for his absence on holiday, saying that though his holiday had been arranged "some time ago," the fires and deaths had "understandably caused a lot of anxiety."

"I deeply regret that and so does (his wife) Jenny and to all those who were affected by the fires especially, we deeply regret it," he said.

Fighting fires in record-breaking heat

One of the worst blazes is the Green Wattle Fire in the Wollondilly Shire region -- where Buxton is located, and where the two firefighters died. Images from the region show thick plumes of smoke blanketing neighborhoods and a hazy orange sky. The fire is more than 167,000 hectares in size and is "out of control," said the RFS , which warned residents in the area that "it is too late to leave."

An Australian firefighter in the town of Bilpin, outside Sydney, on December 19, 2019.

Around 40 homes in Wollondilly Shire were destroyed by the fire on Thursday alone, Nine News reported. That brings the total of destroyed homes to more than 800 this bushfire season, while nearly 300 more have been damaged, according to RFS estimates.

Further north in Wollemi National Park, an even bigger fire rages, consuming 444,000 hectares of land. As of Thursday, there are more than 2,500 firefighters on the ground across the state, working to bring the flames under control, the RFS said.

The bushfires have been burning for two months now, exacerbated by strong winds that stoke the flames and spread dangerous embers, and by a record-breaking heat wave.

Wednesday broke the record for the hottest day nationwide, with the average maximum temperature hitting 41.9 degrees Celsius (107.4 Fahrenheit). That beat the previous record of 40.9 Celsius (105.6 Fahrenheit) -- which had been set just the day before.

That's just the national average -- the heat has spiked much higher in some places. The South Australian town of Nullarbor hit 49.9 degrees Celsius (121.8 Fahrenheit) on Thursday, the highest Australia December temperature on record and the fourth highest temperature on record across all months.

The heat wave began in South Australia earlier this week, but is now spreading east to NSW and Victoria. Conditions in NSW are expected to ease slightly today, before becoming hotter and drier on Saturday, raising the fire risk. A total fire ban statewide remains in place until Saturday midnight.