A river is the only thing stopping three major fires spanning two states merging into a terrifying 'mega-blaze' as locals are told to urgently flee.

As of this morning there's only about 10km of space that stands between Victoria's Corryong blaze and the two fires burning out of control at NSW's Kosciuszko National Park.

Despite the threat, up to half of the residents living between the Upper Murray blaze, and fires burning nearby in NSW, had chosen to stay.

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Incident Controller Leith McKenzie told reporters she was "very frustrated" at the situation.

"They're stubborn and they don't want to move," she said. "They're defending their life and that's what they know."

It comes as NSW Premier Gladys Berejiklian announce there are two people missing in a remote part of the state's fire zone.

At a press conference this morning she said: "Unfortunately, overnight, it has become apparent we have two people unaccounted for in New South Wales.

"I hope that ends in good news but currently in remote parts of far New South Wales there are two people unaccounted for. We hope that ends in good news. At this stage, we can't account for their safety."

A firefighting crew battles a fire near Burrill Lake. Photo / AP
A firefighting crew battles a fire near Burrill Lake. Photo / AP

More than 4.9 million hectares of NSW bush land has now been burnt, and it will soon surpass 5 million, says the commissioner of the New South Wales Rural Fire Service.

"Unfortunately, I don't think it will be too long before we are up over 5 million hectares of largely forestry country, burning along the Great Dividing Range from the Queensland border to the Victorian border," Shane Fitzsimmons told reporters this morning.

'Horrendous weekend'

After a "horrendous" weekend of bushfires on NSW's South Coast, the Southern Highlands and the Snowy Mountains, there are still 136 fires burning in the state.

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Thousands of Australian Defence Force personnel will descend on bushfire-ravaged towns today as hundreds of people are expected to find out their homes have been destroyed.

In South Australia, a convoy of army vehicles with up to 100 reservists is heading for Kangaroo Island as the bushfire recovery gathers pace.

The blaze, which has burnt more than 155,000 hectares inside a 300-kilometre perimeter, is still active in some areas.

In Victoria, showers are giving firefighters some relief but hot conditions are forecast to return later in the week.

More than a million hectares have burnt statewide, 800,000 of them in East Gippsland alone.

Premier Daniel Andrews confirmed four Victorians are still unaccounted for in fire-affected areas.

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Thick smoke descends on Melbourne

The heavy, acrid smell of bushfire smoke from eastern Victoria is finding its way into homes in Melbourne, and it is likely to hang around all of Monday.

The smoke, from fires burning more than 280km away in East Gippsland, is being carried by east to southeasterly winds, according to the Bureau of Meteorology.

The Australian town of Eden was evacuated as fires raged towards it. Photo / news.com.au
The Australian town of Eden was evacuated as fires raged towards it. Photo / news.com.au

The Environmental Protection Authority says the air quality forecast for much of Monday is "very poor".

Residents in the Latrobe Valley, Geelong and Greater Melbourne are being urged to ventilate homes and minimise time spent outside.