'Size of Alaska’: Dire map reveals critical 24 hours ahead

A daunting map reveals the sheer scale of the area set to be pummelled again by relentless rain in Australia's east.

Over half of NSW, including its entire coastline, large parts of southern Queensland and southeastern Victoria, are all subject to a severe weather warning on Tuesday.

A map released by the Bureau of Meteorology (BoM) before 3am shows the giant area stretching at least 1700km through the three states.

Late on Monday the BoM said the warning zone, which stretched across all states bar Western Australia, was similar in size to the expansive US state of Alaska.

Thanks to a significant trough moving towards the coastline and merging with another major system, the weather warning stretches close to the South Australian border in northern NSW and Queensland's south.

NSW Premier Gladys Berejiklian said the conditions could force a further 15,000 people to be evacuated in the state on Tuesday.

BoM flood manager Justin Robinson said the conditions are the worst he has seen in his 20-year career at the bureau.

"We've got a flood watch that covers all the way from the Queensland border down to the Victorian border - all those coastal rivers," he said.

Sydney and the Mid North Coast, which have faced the brunt of the conditions in recent days, could see another 100mm on Tuesday, while a season's worth of rain is possible in the state's west.

Some areas of the Mid North Coast have copped more than a metre of rain over the last week. Kempsey, one of the worst hit places on Monday, has received more than 600mm in the past 4 days.

Up to 300mm still to hit NSW's south coast

The worst hit on Tuesday will be the state's south coast as the conditions stretch below Sydney.

Up to 200mm of rain is expected across the region while some parts could see 300mm.

Further evacuations have been issued for residents northwest of Sydney as the 3,000 residents evacuated in the region up to Monday expected to grow significantly.

• 'Apocalypse state': New disaster wreaks havoc year after horror

• Shocking aerial photos reveal town transformed by flooding

• 'Skin-crawling' video shows spider infestations in NSW floods

• 'Kick in the guts': Family homes lost to flood after bushfires battle

• 'Brace yourselves': Premier's confronting warning amid flooding

The Hawkesbury river continues to rise while major flooding of the Colo River prompted an evacuation warning, with residents in the area told to leave by 5am.

The Hawkesbury reached close to 13 metres at Windsor, the historic town that was concerning authorities on Monday.

Communities such as Portland and Wisemans Ferry are likely to be badly affected as the Colo River flows into the Hawkesbury.

The Prime Minister's office confirmed late on Monday the Australian Defence Force will be sent in to assist at the request of the NSW government.

More than 18,000 people were evacuated up to Monday across the state, with further evacuations expected.

Head of Resilience NSW Shane Fitzsimmons told ABC Breakfast the developments are "terribly confronting".

"In New South Wales this is just another disaster, another setback, in what has been a really challenging 18 months to two years.

Premier Gladys Berejiklian warned the state to prepare for the conditions continuing into Tuesday.

"We're not through the worst of it potentially and that's why we need to brace ourselves," Premier Gladys Berejiklian said on Monday.

"We have no illusions about how difficult the next few weeks and months will be."

The SES said on Monday reintroducing essential services such as electricity and water to some homes could take months.

Mid North Coast police say thousands of homes have suffered from flooding.

More to come.

Do you have a story tip? Email: newsroomau@yahoonews.com

You can also follow us on Facebook, Instagram and Twitter and download the Yahoo News app from the App Store or Google Play.