Sydney's desalination plant has delivered an unexpected effect on the marine life surrounding its discharge outlet — a massive increase in fish life.

Key points: A study finds excess hypersaline water pumped off Sydney increased fish life by 279 per cent

A study finds excess hypersaline water pumped off Sydney increased fish life by 279 per cent Researchers say it's not what they were expecting to find

Researchers say it's not what they were expecting to find The study concluded that large-scale desalination does not negatively affect fish numbers

Research led by Southern Cross University and published today in the journal Environmental Science & Technology has found the excess hypersaline water pumped at the outlet off Kurnell increased fish life by 279 per cent when the plant was in operation.

Lead researcher from the university's National Marine Science Centre Professor Brendan Kelaher said the results were not what they had been expecting when they began the study.

"We went into this study reading the global literature and there was a general feeling that there may be no impact or maybe a decrease in fish life," Dr Kelaher said.

"We were really surprised to find such strong positive effects.

"They turned on the plant and there were fish everywhere. It was really quite amazing."

Sydney's desalination plant is one of the largest in the world and opened in 2010. ( Supplied: John Holland )

The study, jointly conducted by SCU and the University of New South Wales, observed the area of ocean subject to the discharge outlet both before and after the desalination plant's initial start-up in 2010, as well during a period when the plant temporarily ceased operations.

The general increase in fish numbers at the site included a 133 per cent increase in fish targeted by commercial and recreational fishers within a 50-metre radius of the outlet.

But one thing the research did not look at was what caused the influx of fish when the plant was pumping the super-saline water into the ocean.

"More research is needed to understand it fully, but there really wasn't any evidence for an increase in food, so we think it was the disturbance created by the mixing of the brine," Dr Kelaher said.

"Disturbance often comes with increased food availability in the ocean and these fish were just attracted to that turbulent water thinking that something more was going on."

Dr Kelaher said it was likely the increase in numbers was simply fish moving to the site, but there could also be an increase in breeding in the vicinity.

"Based on all of the evidence from our control sites, we didn't see a decrease in abundance in those places, so possibly it's related to production, but a little more research would help us out," he said.

Southern Cross University lead researcher Professor Brendan Kelaher was surprised to find an almost three-fold increase in fish numbers. ( ABC Coffs Coast: Melissa Martin )

The desalination plant is one of the largest in the world and produces up to 15 per cent of Sydney's drinking water.

Sydney Water initially funded the study, but after it was sold the funding was continued by the Sydney Desalination Plant.

The study was a condition of the development approval of the plant and was overseen by the Office of Environment and Heritage, DPI, and a panel of three independent experts.

University of Melbourne Professor of Marine Biology Mick Keogh is one of those independent experts and has been equally surprised by the findings of what he described as "rigorous research".

"I guess with desalination, people expect there to be negative effects, but in this case what was happening was big increases in a lot of fish," Professor Keogh said.

"It's not too surprising when you build a structure on the sea floor that fish are attracted to it, but the surprising bit was that other kinds of fish only appeared when you turned it on and I don't think anyone was expecting that."

The results come at a time of growing pressure on potable water supplies, but the study concluded that well-designed marine infrastructure and discharge from large-scale desalination would not negatively affect local fish abundance or species richness.

It also found that, if appropriately managed, it could enhance local fishing opportunities.