Bushfire activity across south-eastern Australia has picked up as the region is swept by hot, gusty winds that authorities fear could render blazes unstoppable.

Key points: Temperatures have reached into the mid-40s in parts of NSW and Victoria as strong winds fan bushfire activity

Temperatures have reached into the mid-40s in parts of NSW and Victoria as strong winds fan bushfire activity Authorities fear existing bushfires will spread and join and dry lightning could start new ones

Authorities fear existing bushfires will spread and join and dry lightning could start new ones The NSW Rural Fire Service says fires could move "frighteningly quickly" like they did on New Year's Eve

In New South Wales, emergency warnings have already gone out for several communities who are being hit by bushfires, which have closed a stretch of the Princes Highway south of Nowra.

Further south, six evacuation notices and about a dozen emergency warnings have been issued as bushfires threaten communities in Victoria's north-east and east.

Authorities are also concerned that the weather front sweeping the region could result in dry lightning strikes, starting new fires across parts of New South Wales and Victoria that have not already been devastated over recent days.

Dozens of communities in East Gippsland are being threatened by bushfires. ( Supplied: Australian Maritime Safety Authority )

Temperatures in parts of each state have already climbed into the 40s, with Albury-Wodonga reaching 45 degrees Celsius by 2:00pm.

Victoria's Police and Emergency Services Minister Lisa Neville has identified the second person to die in the state's fires as Fred Becker, a timber worker from Maramingo Creek, near Genoa in East Gippsland.

Victoria Police Chief Commissioner Graham Ashton said the number of people missing now stood at 21, seven fewer than the number unaccounted for 24 hours earlier.

Meanwhile, South Australian Premier Steven Marshall has confirmed two people have died in the bushfires on Kangaroo Island.

The fire has burnt up to 150,000 hectares, including most of Flinders Chase National Park.

Mr Marshall expressed his condolences and said next of kin were being notified.

Almost 60 evacuees from the fire-hit Victorian coastal town of Mallacoota have arrived at Hastings, south-east of Melbourne, on the Navy training ship MV Sycamore.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 58 seconds 58 s The MV Sycamore was carrying 58 people, two dogs, a cat and a rabbit.

About 1,000 more people are expected to arrive on another Navy ship, HMAS Choules, later today.

Across the state around 100,000 people were urged to evacuate ahead of today's dangerous conditions.

Fires have burned about 820,000 hectares across Victoria in recent days, and about 50 blazes are continuing to burn in the state.

"We can expect that with the winds this morning and the higher temperatures followed by this change later in the day, south-westerly change, that the fires will be quite unpredictable in their behaviour and spread and that's going to make firefighting difficult," State Control Centre spokesperson James Todd said.

Evacuees gather at Batemans Bay as fire burns close to the coastline. ( Supplied: Mark Cuddy )

'We've never been as prepared,' NSW Premier says

Across the border in NSW, thousands more heeded warnings to leave large parts of the NSW South Coast, from Nowra to the Victorian border and the Snowy Monaro region.

NSW Premier Gladys Berejiklian said on Saturday morning there were 137 bush and grass fires burning across the state, with about 60 uncontained.

More than 3,000 firefighters will be on the ground in NSW today.

"We've never been as prepared as we are today for the onslaught we're likely to face due to the deteriorating conditions," she said.

"Today is all about saving lives," she said.

Fires 'spreading quickly' across southern NSW

There are several fires burning at Emergency Warning level in the Snowy Mountains, close to the NSW-Victoria border.

Closer to Sydney, the Green Wattle Creek blaze is also threatening properties.

Numerous other fires are listed at "watch-and-act" level, stretching from the Kerry Ridge fire north of Sydney to Werri Berri, south of Bega.

The RFS says "several fires are spreading" in the Kosciuszko National Park.

A "temperature inversion" that has been keeping conditions cool across NSW has started to lift, with rising rising mercury and increasing winds causing headaches for firefighters.

Fires expected to spread in NSW

A total fire ban is in place across NSW with extreme fire danger in much of the state's south-east, and authorities are warning conditions could become catastrophic.

Existing fires are expected to spread significantly under difficult conditions.

An RFS map of predicted fire paths shows a blaze at Green Valley could impact nearly the entire Mount Kosciuszko National Park with either actual fire spread or ember attacks.

The NSW RFS issued this map showing the predicted fire spread for January 4. ( ABC News )

RFS Deputy Commissioner Rob Rogers said the fires could move "frighteningly quickly", as they did on New Year's Eve.

"Even those areas that have already been impacted by fire, there's still a lot of pockets among the bush that we're quite concerned about that might flare up," Mr Rogers said.

He pleaded with people to avoid the path of fires and head to a larger town or beach for safety.

"Our pure focus … is about the preservation of life."

Mr Rogers warned a stretch of fire that has crossed into Victoria could return on Saturday, potentially merging with another blaze and creating a fire front stretching up to 70 kilometres.

Locals wait for a community meeting in the town of Narooma on the NSW South Coast. ( ABC News: Jonathan Hair )

"We can't stop those fires. We can't stop the fires we already have."

Closer to Sydney, another point of vulnerability is on the southern side of the Green Wattle Creek Blaze burning near Mittagong.

"Our strategy is to lay retardant on it ahead of it just to try and slow its progress, so it doesn't get to places like Mittagong," Mr Rogers said.

Some in Mallacoota were forced to flee the flames via water. ( Supplied: Via Twitter @Nic_Asher )

The RFS is also closely watching a section of the Gospers Mountain blaze, north-west of Penrith, where predictions suggest fire and ember attacks could creep towards Sydney.

Mr Rogers said there was a "breakout" there on Friday and while crews were confident of getting on top of it, the blaze remained a concern.

Huge fire fronts raging across Victoria

In Victoria, a huge complex of fires in East Gippsland stretches from north of Bairnsdale to the New South Wales border, with areas not already burnt under threat.

Another complex of fires in the Alpine region, started by dry lightning this week, is threatening a number of ski resorts and could join up with a third group of fires burning in the north-east of the state, around Corryong and Walwa.

Scores of properties or structures have been destroyed in Victoria. ( AAP Image )

There are also serious fires burning near Portland in the state's south-west, in the Budj Bim National Park and in pine plantations at Wade Junction.

Victoria's Emergency Management Commissioner Andrew Crisp said the state had "literally hundreds and hundreds of kilometres of active fire edge, of uncontained fire".

"The major risk is the fires we don't know about, the new starts," he said.

"Crews will be so busy protecting communities that fighting new fires in dangerous conditions will be very difficult."

The ranks of Australian crews will be bolstered by the addition of 41 firefighters from the United States today, and up to 70 aircraft will be used to attack the blazes.

Authorities are urging people across affected areas not to become complacent and to expect an escalation in bushfire behaviour later in the day.

The Bureau of Meteorology is expecting hot, dry north-westerly winds ahead of a southerly change.

The change has already reached Victoria's south-west and crossed metropolitan Melbourne about 8:00am.

It will push into the Gippsland region — the area most affected by the recent fires — by the early afternoon and will move into the north-east of the state by evening and later on into NSW.

Temperatures on Sunday are expected to drop dramatically, into the high-teens and low-20s.