Six million litres of untreated sewage and stormwater has spilled into Hobart's River Derwent after a power failure at the city's treatment plant.

Key points: The latest incident follows a spill on New Year's Eve last year, in which 10 million litres of sewage leaked into the river

The latest incident follows a spill on New Year's Eve last year, in which 10 million litres of sewage leaked into the river TasWater sought to reassure the public by saying the sewage flowed into a "very large … very deep part of the river"

TasWater sought to reassure the public by saying the sewage flowed into a "very large … very deep part of the river" Tests are being undertaken "far and wide", with results expected by Tuesday, TasWater said

Hobart City Council and the Environment Protection Authority are investigating the impact of the spill on the environment, while TasWater is examining what happened TasWater's Macquarie Point Sewage Treatment Plant on Sunday.

The treatment plant returned to operations on Monday.

TasWater acting regional services department manager Brendan Hanigan said the results from water sampling would be released on Tuesday.

"Our initial investigations are there is very good dispersal across the Derwent River from this particular outfall," Mr Hanigan said.

"It is a very large part of the river, it's a deep part of the river, we've had no signs reported to us of any visible impacts — but to confirm this, we've undertaken a series of samples north of the outfall."

TasWater say there has been "very good dispersal across the Derwent River". ( ABC News )

Avoid swimming, health watchdog says

Water was tested north of the Tasman Bridge, at Eastern Shore beaches and near the CSIRO, Mr Hannigan said.

"We are casting our sampling net far and wide to see if there are any indications and by tomorrow we should have results of those sample tests to confirm if we can find any contamination as a result of this incident," he said on Monday.

TasWater has committed to implementing any recommendations from its review of the incident.

Ten million litres of sewage was pumped into the River Derwent from the same treatment plant on December 31 last year.

That incident was caused by a 24-hour failure in the plant's effluent chlorination process.

In a statement, a Public Health Services spokesman said the agency "recommends avoiding swimming in recreational waters for five days after any sewage spill or heavy rainfall".

The ABC has contacted the Environment Protection Authority for comment.