Concerns have been raised about the future quality of Sydney's drinking water due to the loss of senior scientists who oversee standards.

WaterNSW has confirmed six jobs have been axed from the organisation, with four of the staff based at Penrith in Sydney's west taking voluntary redundancy.

The organisation said the changes would not affect its ability to monitor and protect the city's water supplies.

However the New South Wales Opposition has warned the quality of Sydney's drinking water could be jeopardised.

Labor's water spokesman Mick Veitch has described the redundancies as a major loss.

"These are people who have years and years of experience," he said.

"They know the science, they understand the science around Sydney's water quality, our drinking water."

Mr Veitch said the city did not need a repeat of the 1998 water quality scare when a number of people fell ill because of contamination by microscopic pathogens, such as cryptosporidium and giardia.

"This regime was put in place, with scientists of this calibre, to ensure that that doesn't happen again, " Mr Veitch said.

Tony Webber from WaterNSW told 702 ABC Sydney the organisation was not facing the scale of cuts that had been reported.

"You need the best of the best and our position is that nothing has changed," Mr Webber said.

"We've had two or three people take a VR [voluntary redundancy] but we're spending an extra $600,000 in the next financial year on water quality science.

"We are still engaging in public education, we're still doing the compliance, the land management, the monitoring, the environmental controls that we've always done, and that's how you reduce the risk to the city's water supply."

Remaining staff placed under greater pressure, Labor says

Mr Veitch also said the cuts would place pressure on the remaining staff at Penrith.

"The staff that remain have got the do the same amount of work, have got to do the same amount of monitoring and it's going to take the same amount of scientific rigour without their senior colleagues," he said.

"The reality is that Mike Baird and Niall Blair, the Minister, have got to tell the people of Sydney that this will not impact on the quality of our water," Mr Veitch said.

The Water Minister, Mr Blair, has said any structural changes will not diminish efforts to protect Greater Sydney's drinking water catchments and deliver high quality water.

Associate Professor Stuart Khan said job cuts at WaterNSW could mean that the state may not be prepared if the unexpected happens. ( ABC News, file photo )

According to the Minister, the redundancies were designed to achieve greater efficiency, reduce costs and enhance frontline services.

The Minister's office also said the amalgamation of the Sydney Catchment Authority and the State Water Corporation, to create WaterNSW, would not impact on water monitoring and catchment protection programs that were core functions.

WaterNSW added that the protection of the catchment was enshrined in legislation.

Associate Professor at the School of Civil and Environmental Engineering at University of NSW, Stuart Khan said the cuts would mean if something unforeseen happened the state may not be equipped to deal with it.

"I think we can be confident that the next time something unpredictable, like that cryptosporidium incident [in 1998], comes about again, we will be in the same position we were in 1998," Associate Professor Khan said.

"Where we were scrambling like a bunch of buffoons because we didn't know how to deal with that crisis.

"We didn't understand the organism, where it came from or how to manage it in drinking water.

"And we'll be back in that situation again."