The Water Corporation believe they accidentally killed nearly one hundred fish after discarding contaminated water into a small lake in South Perth.

From last Friday a lake on Mill Point Road was being used by the Water Corporation as a dumping ground for excavated groundwater, dug up as part of routine maintenance works.

But four days after works began about 80 black bream fish washed up dead and initial studies indicate excess acid in the groundwater may have stripped oxygen levels in the pond, the Water Corporation said.

Lake water drains into the Swan River near the Narrows Bridge and officials fear some of the contaminated  groundwater may have also escaped into the river before drainage channels were blocked.

Works at the site, which were expected to take about three weeks, have now been suspended while further investigations are carried out by both the Water Corporation and The Swan River Trust.

Prosecution under the Environmental Protection Act may be carried out, a spokesperson from The Swan River Trust said.

A spokesperson from the Water Corporation Steven MacKenzie said this type of thing has never happened before.

“The Water Corporation had a permit from the Swan River Trust to dispose of the groundwater to the pond, but the impacts of the disposal were underestimated,” he said.

“The Water Corporation had considered disposal to sewer, road drainage, the pond and directly into the river.

“In this case, the sewer and road drainage did not have sufficient capacity to take all of the dewatering water, and disposal to the pond and directly to the river were considered to be environmentally acceptable disposal locations.

"As the pond was closest, it was chosen as the preferred disposal location.''

The department of Fisheries fish health laboratory is analysing fish samples and the Swan River Trust said it’s too soon to know what the environmental impact on the lake and river will be.

The Water Corporation believe it is unlikely the river would be affected by the incident as the contaminated groundwater will be massively diluted by such a large flowing water body.

Sue Oldham from the Department of Health said people are advised not to swim in the lake until investigations reveal the exact cause of the fish deaths.