Coral bleaching has been discovered in Sydney Harbour for what is believed to be the first time.

Marine biologists found up to 45 per cent of corals at certain sites were bleached.

The team from the University of Technology in Sydney (UTS) and Macquarie University believed above-average sea temperatures caused by an El Nino weather event caused the damage.

Dr Matthew Nitschke from UTS said while the coral was expected to recover, it was worrying to see that type of bleaching in the harbour.

"It's certainly extensive enough to cause some concern," Dr Nitschke said.

"It's kind of constricted to the upper surface of boulders which to us tells us it's a combination of high light levels and also bleaching, so maybe we had some clean water that came through at the same time.

"It's enough to cause us some concern and say that we think this is a pretty significant bleaching event for Sydney Harbour."

Dr Nitschke said the water temperatures had now dropped and the coral was not expected to sustain any long-term damage.

The team will monitor the corals during the next few months to assess its recovery.

A coral bed in Sydney Harbour in the process of bleaching. ( Supplied: UTS )

"This is the first time that we think this has happened in Sydney Harbour, [so] theoretically they should be in pretty prime condition," he said.

"It could be a year in isolation because we had this El Nino event which is occurring across the Pacific.

"We do expect that this is something that could continue into the future."

State Government does not have climate plan: Labor

The New South Wales Opposition's environment spokeswoman Penny Sharpe said it was a disturbing development that proved climate change was in effect in Australia.

"All the warning signs have been there that climate change is going more rapidly than we first thought," she said.

"Scientists have been warning about this and this is the first proof that it really is.

"This is not just a matter for the reef and for some other country to deal with.

"Climate change is here and the NSW Government doesn't have a plan to address it."