Authorities fear there's been "significant damage and destruction" across NSW and Victoria with dozens of buildings potentially lost as a number of bushfires continue to create emergency conditions across the two states.

Overnight, the Rural Fire Service advised "fires were spreading quickly" particularly in southern New South Wales around Bega, Shoalhaven and the Southern Highlands close to Sydney.

In NSW there are currently three emergency warnings in place for out of control fires. Some 143 fires were burning across NSW on Saturday night with 3600 firefighters on the ground.

Rural Fire Service Commissioner Shane Fitzsimmons said they were receiving reports of "significant damage and destruction" on Saturday night after more than a dozen blazes reached an emergency warning alert level throughout the day. He believed property losses could run into the dozens.

More evacuation orders were issued to Victorians overnight as massive blazes continued to ravage the state's east. Dozens of fires were burning in Victoria early on Sunday, 13 of which were subject to emergency warnings, the highest alert level. Six people are missing.

Four Fire & Rescue NSW firefighters were injured overnight and another person has died as a result of the bushfires in NSW, with a man suffering a heart attack while trying to save his friend's home, Premier Gladys Berejiklian has confirmed.

"Very sadly one person lost their life due to a cardiac arrest just outside of Batlow last night and police have just issued details about that one deceased person and, of course, our hearts, thoughts and minds go to their families during this very difficult time," Berejiklian said in a press conference this morning.

1:15am Sunday.
Fire crews mopping up spot fires by the Princes Hwy as they head in to fire affected areas South of Eden, NSW. Wind here seems to have changed direction to NE. @abcnews pic.twitter.com/hbjJIngla0

— Greg Nelson ACS (@GregNelsonACS) January 4, 2020

The entire town was considered "undefendable" and its 1300 residents had been told to evacuate ahead of "widespread extreme fire danger".

NSW Police said the man had been trying to help defend his friend's rural property at Batlow, news.com.au reports.

"About 6.30pm yesterday (Saturday), a 47-year-old man was assisting in the effort to defend a home on Batlow Rd when he went to a nearby ute with the intention of retrieving water," police said in a statement on Sunday morning.

"When he didn't return, the man's friend went to check on him and found him unconscious in the vehicle.

"The man's friend got into the ute and drove a short distance, before coming across a passing police vehicle, with officers stopping to assist.

"Despite the efforts of emergency services, the man died at the scene."

NSW RFS Commissioner Shane Fitzsimmons said the man had come down from Goulburn.

"Police and RFS personnel rendered first aid including defibrillation, ambulance arrived but the poor gentleman couldn't be saved," he said.

The premier said "every time we hear of a life lost, it cuts us deeply".

But she was relieved to say there was no one unaccounted for across the state "at this stage" on Sunday morning.

"That is a huge, huge relief," Berejiklian said.

She said the "immediate focus" of authorities today was not just to contain and control fires and protect life, "it's also to focus on recovery".

FRNSW Commissioner Paul Baxter gave an update to Nine's Today about the health of the four firefighters injured overnight, three of whom were out battling bushfires.

"(This is) a real example of the extreme conditions that our firefighters faced last night," he said.

"Two of the firefighters were doing the really important work of protecting critical infrastructure at the Milton waterworks fire down south and they've suffered heat exhaustion and severe smoke inhalation.

"They've been treated by (NSW) Ambulance, both have been able to be released overnight.

"We had another firefighter in the Hunter Valley also under those extreme conditions with severe smoke inhalation as well who was also treated by Ambulance and released.

"In an unrelated matter from Wentworthville, a firefighter suffered severe burns to his hands dealing with a house fire.

"Obviously really concerning to us, firefighter safety."

The deadly wildfires, which have been raging since September, have already burned about 5 million hectares of land and destroyed more than 1500 homes.

The early and devastating start to Australia's summer wildfires has also been catastrophic for the country's wildlife, likely killing nearly 500 million birds, reptiles and mammals in New South Wales alone, Sydney University ecologist Chris Dickman told the Sydney Morning Herald. Frogs, bats and insects are excluded from his estimate, making the toll on animals much greater.

Experts say climate change has exacerbated the unprecedented wildfires around the world. Morrison has been criticised for his repeated refusal to say climate change is impacting the fires, instead deeming them a natural disaster.

Some residents yelled at the prime minister earlier in the week during a visit to New South Wales, where people were upset with the lack of fire equipment their towns had. After fielding criticism for taking a family vacation in Hawaii as the wildfire crisis unfolded in December, Morrison announced he was postponing visits to India and Japan that were scheduled for later this month.

The government has committed A$20 million to lease four fire-fighting aircraft for the duration of the crisis, and the helicopter-equipped HMAS Adelaide was deployed to assist evacuations from fire-ravaged areas.