The NSW Environmental Protection Authority is investigating Sydney Water after large amounts of raw sewage spilled into the Parramatta River last week.

Key points: The spill happened on October 21 after a wastewater pumping station collapsed

The spill happened on October 21 after a wastewater pumping station collapsed Authorities have warned people not to swim or fish between Northmead and Silverwater Bridge

Authorities have warned people not to swim or fish between Northmead and Silverwater Bridge Sydney Water is pumping fresh water into the river and other measures to minimise damage

A large volume of the untreated waste water polluted the Parramatta River on October 21, when a wall in a waste water pumping station at Northmead collapsed.

The overflow has now stopped and bypasses are in place while Sydney Water replaces the pumping station and conducts an investigation.

Gary Hurley from Sydney Water apologised to local residents and said they were still working out how much sewage went in the river.

However, he did say it was a "substantial flow".

The overflow was stopped within 24 hours but there have been leaks since.

"We've certainly had a number of fish killed, they were mainly carp, and we'll work with the local authorities to restock that with more native species," Mr Hurley said.

The collapsed pumping station which sat underneath the double brick structure. ( ABC News: Kathleen Calderwood )

The above-ground structure of the station — about the size of a two-storey house and made of double brick — has been removed, and work has started a few metres away to build a temporary replacement which Sydney Water plans to have operational by around Christmas.

"We've had a structural failure below the ground, cause is unknown, but that's resulted in a lot of instability in the structure and it actually caused damage to the pumps so the station became inoperable," he said.

"Crews have worked around the clock for the last week to make the site safe … and to minimise the impact on the environment and the local community."

Mr Hurley said there was no indication of the cause of the collapse and the station had recently been inspected.

'This is old technology'

The spill occurred days after the Parramatta River Catchment Group (PRCG) — of which Sydney Water is a member — launched its plan to open three new swimming spots along the river by 2025.

PRCG chair Councillor Mark Drury said the incident was very concerning and he wanted to ensure a similar incident did not happen in the future.

"We do have old infrastructure along the way and it needs to be maintained," he said, adding that he hoped Sydney Water would have a systematic review of its pump stations.

"We've got to get a lot smarter about this, we can't have sewage that effectively goes into our waterways.

"All around the world they've got better ways of doing it, this is old technology, probably put in at a time where we didn't care about the river."

Ian Wright from Western Sydney University said there could be public health and environmental consequences.

Water from the Parramatta River is aerated. ( ABC News: Kathleen Calderwood )

"I can see Sydney Water are throwing everything at this, as they should — it's going to be a slow, expensive, difficult process," Dr Wright said.

"I reckon this is going to take quite a long time for the natural environment to absorb the impact."

In a statement, the NSW Environmental Protection Agency (EPA) said it was working with government agencies following the incident, and its investigation was ongoing.

The Western Sydney Local Health District (WSLHD) is working with Sydney Water and has directed residents to avoid swimming, fishing and other recreational activities between Northmead and the Silverwater Bridge at Ermington.

Pets should also avoid swimming in the river, they advised.

WSLHD said their public health unit is reviewing sample results from the river on a daily basis and water quality has improved over the last few days.

"Although Sydney Water is taking all possible steps to reduce the risk of a future overflow, there is the potential for further sewage overflows whilst Sydney Water is working to reinstate the Wastewater Pumping Station at Northmead."

Lake Parramatta is safe to use as it is upstream of the overflow area.

Sydney Water is flushing the area with fresh water, as well as using aerators to increase oxygen in the water.