The National Parks and Wildlife Service has warned anyone on the water around Sydney's Middle Harbour to be aware of a southern right whale in the area.

The juvenile whale spent Saturday frolicking around Balmoral.

But Parks and Wildlife Service co-ordinator of Marine Fauna Programs Geoff Ross said the whale would not be monitored overnight.

"During the night time it's most important for all vessel users who are using the harbour to pay particular attention, particularly if they're coming and going from The Spit or via Middle Head," he said.

"Right whales are very hard to see and one inside the harbour does present an issue for vessel collisions."

The southern right whale delighted Sydneysiders as it ventured into the suburbs.

It spent several hours in the inner harbour swimming close to shore at Chinaman's Beach and around The Spit.

Kayakers and boaters were able to get close enough to see the barnacles and get a spray when it came up for breath.

The whale is thought to be the same whale spotted off northern beaches recently.