Thousands of people have fled into the water at a beachside town in Australia to seek shelter from a massive bushfire bearing down on the area.

Residents said the blaze moved into the Victorian town of Mallacoota on Tuesday morning, throwing embers towards homes.

Online, people reported the "roar" of the fire and posted pictures of a black and then deep-red sky.

Several popular holiday spots along the coast between Sydney and Melbourne are currently under threat from bushfires.

Authorities had told people in these regions - many of them tourists - to stay put because it was too late and dangerous to evacuate.

Victoria's Premier Daniel Andrews said authorities held concerns for four people missing in the region.

Crews are fighting to protect lives rather than buildings, locals said. Several homes in the town are already believed to have been destroyed.