Tasmania's second largest city, Launceston, is on high alert as floodwaters approach and residents evacuate low-lying areas.

Major northern river systems have risen from record rain after a deadly deluge that has battered three states.

Key points: Two people remain missing

Two people remain missing Longford residents advised to evacuate

Longford residents advised to evacuate Waters expected to peak between 2:00am and 10:00am

Waters expected to peak between 2:00am and 10:00am Spirit of Tasmania services on hold

About 3,000 residents and 800 businesses have been advised to evacuate in the low-lying Launceston suburb of Invermay.

Police are also pleading with Tasmanians to be careful around floodwaters as the search continues for two missing men.

An elderly man is still missing from Ouse in the south, and the search continues for a man whose car was swept away by rising floodwaters at Evandale in the north.

A woman who was in the same car was rescued — one of 100 rescues in flood-affected zones. Of the rescues, 20 people were winched to safety by helicopter.

Earlier police recovered the body of 75-year-old Mary Alford from her home at Latrobe in the state's north-west.

Her husband lost contact with her before being rescued off the roof of their property yesterday morning.

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A flood evacuation centre has been set up at the Silverdome at Prospect and at the University of Tasmania at Newnham.

The Launceston Flood Authority is confident new flood levees finished last year will protect the city.

"We have constructed them to a one-in-200-year flood level; the predicted level from this flood is somewhere between the one-in-50, or one-in-100," general manager Andrew Fullard said.

"The Bureau of Meteorology has predicted the flood peak at 8:00am tomorrow [Wednesday] morning with a high tide at about 4:00am as well which will impact the levels."

Sorry, this video has expired Thousands told to evacuate as rivers rise in Launceston

Mhairi Revie from the SES said people should not be complacent.

"The levees are still levees and levees can fail," she said.

A flood siren at Invermay would sound if residents were in immediate danger.

"If they do hear the siren, absolutely that is time to leave and that will only occur with a decision that we want them to leave now and at that point we'll be doing other public information," she said.

At Longford, just to the south of Launceston, residents are being told to consider leaving as a precaution as the threat from floodwaters fed by the South Esk River continues to grow.

Damage bill expected in the millions

In a life-threatening emergency call 000

In a life-threatening emergency call 000 Call your local SES on 132 500

Call your local SES on 132 500 Bureau of Meteorology

Bureau of Meteorology ABC Emergency

In most areas the damage bill is still being calculated with farmers in particular expected to be hit hard.

Thousands of head of stock across the state have been lost and Premier Will Hodgman said the cost was expected to keep rising.

"There is no doubt that there will be an extraordinary cost and it is catastrophic," he said.

The Government has offered emergency aid for those who need it.

Major flood warnings continue for the Meander, South Esk and Macquarie Basins.

Dangerous conditions in the Mersey River have stranded hundreds of Spirit of Tasmania passengers in Devonport and Melbourne.

The TT-Line expects to resume sailings on Sunday and is negotiating to use ports at either Burnie or Bell Bay.

Danger not over as waters recede

More than 100 roads are closed across the state and many bridges are damaged.

The SES is advising people not to travel unless necessary.

In the south and north-west flood waters are starting to recede but police said the danger was not yet over.

Flood gates have been erected around Launceston including on the Charles Street Bridge. ( Facebook: Adam Markham )

At Latrobe, one of the worst hit areas, some residents have been returning home to assess the damage.

Some have lost everything and many said the warning came too late.

Resident Sam Cunningham is waiting on an insurance assessment to find out how much her family will be out of pocket.

"By the time we got woken up we opened the front door to see what the water was like, then we went up stairs and went up the back and we realised it was much worse," she said.

"It was actually coming from behind us and the water was rushing through the house. We just got the dogs and the kids and we left, we couldn't take everything."

Senior forecaster Glenn Perrin said some of those cleaning up in the state's north could experience minor flash flooding in the coming days.

"The next sort of danger period at this stage is late Wednesday into Thursday, so the rain ... may be locally about 20 or 30 millimetres," he said.

"Now that will be falling in the areas where we've had flooding issues."

Treatment plant outages prompt health warning

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Health authorities are warning people risk illness from contact with flood waters, which may be contaminated with raw sewage.

Flood damage and power outages have also affected some water and sewerage treatment plants.

About 1,000 homes and businesses are expected to spend the night without power.

Prime Minister Malcolm Turnbull said federal authorities were in touch with Tasmanian counterparts and Opposition Leader Bill Shorten urged residents to exercise caution.

Amidst severe rains and unprecedented flooding a wooden bridge in central Tasmania, which survived the flooding, was destroyed by fire.

Forestry Tasmania said the fire appeared to have been deliberately lit and police are investigating.