Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, were in a truck convoy fighting blazes southwest of Sydney when a tree fell and caused the vehicle to roll off the road. The two men, both fathers to 19-month-old children, died at the scene while three other firefighters were injured and taken to a hospital.
New South Wales Premier Gladys Berejiklian said the injured firefighters were in stable condition.
Rural Fire Service Commissioner Shane Fitzsimmons spent the night consoling families of the victims.
"To not be coming home after their shift is a tremendous grief and I applaud the families and the loved ones for their remarkable comprehension of what's been unfolding," he told reporters.
"Both of these men were very well respected, they were very close, they're a close-knit brigade, they're a very community-focused brigade, work together, socialize together, they're very interactive together."
Morrison said the two firefighters were "bravely defending their communities with an unmatched spirit and a dedication that will forever set them apart amongst our most courageous Australians".
"Their sacrifice and service saving lives and saving properties will be forever remembered. I wish those injured all the best in their recovery," he added.
The Rural Fire Service said up to 40 houses could be destroyed southwest of Sydney.
Cooler conditions provided desperately needed relief Friday, but scorching temperatures are forecast at the weekend with Sydney's western suburbs tipped to reach 45 degrees Celsius (113 Fahrenheit).
New South Wales declared a seven-day state of emergency Thursday as around 2,000 firefighters battle 100 wildfires across the state. Around 3 million hectares (7.4 million acres) of land has burnt nationwide during a torrid past few months, with six people killed and more than 800 homes destroyed.
The annual Australian fire season, which peaks during the Southern Hemisphere summer, started early after an unusually warm and dry winter.
The Bureau of Meteorology said Wednesday was the hottest day on record in Australia with an average of 41.9 Celsius (107.4 F), beating the landmark set a day earlier by one degree.
Adelaide, in the southeast, is currently in the midst of a heatwave peaking at a sizzling 46 Celsius (115 F) on Friday, while Melbourne was forecast at 44 Celsius (111 F), which would be the Victoria state capital's hottest since the devastating Black Saturday wildfires in 2009.
Melbourne on Friday was shrouded in smoke wafting from the New South Wales wildfires much like the haze that has often blanketed Sydney during the past month, making its iconic skyline barely visible.
The unprecedented conditions has reignited debate on whether Australia's conservative government has taken enough action on climate change. Australia is the world's largest exporter of coal and liquefied natural gas.
Protesters on Thursday camped outside Morrison's Sydney residence demanding urgent action on climate change.
Morrison has copped criticism for going on a family holiday in Hawaii during the wildfires crisis, but said he would cut short his vacation and return to Sydney on Saturday.
"I deeply regret any offence caused to any of the many Australians affected by the terrible bushfires by my taking leave with family at this time," Morrison said. "I have been receiving regular updates on the bushfires disaster as well as the status of the search for and treatment of the victims of the White Island tragedy."