Two very rare southern right whales have been spotted off Sydney's northern beaches as the mother and calf duo make their way into the harbour city before their treacherous journey to the Antarctic.

The whales have made a pit stop on their southward journey and are relatively stationary in shallow waters northeast of Long Reef, whale conservation group ORRCA said.

NSW Parks and Wildlife Service authorities are asking the public to keep at least 300 metres away to avoid disturbing the whales which might cause them to move on, tiring them unnecessarily.

Two southern right whales were spotted off Sydney's northern beaches on Saturday morning, making their way to the shoreline before the pair head to the Antarctic

Quiet, undisturbed time in shallow, sandy bottom bays and protected beaches is a critical stage in southern right whale calf development.

The calf is expected to consume about 300 litres of milk per day to gain the strength before the pair embark on the long voyage towards the Antarctic, according to park ranger Andy Marshall.

'The biggest threat to the survival of southern right whales in NSW waters is disturbance from people getting too close,' he said.

Any disturbance to the whales not only compromises a calf's ability to nurse and develop, but it can also drive them out into deeper waters, where they are exposed to predation by orcas and sharks, the ranger said.

People have been urged to keep their distance from the two whales, with wildlife authorities concerned the attention will affect the whale and its calf from preparing for their journey down south

The pair were first spotted on July 27 in Coffs Harbour, and authorities have been tracking them as they made their way down the coast.

Mr Marshall said the calf was born about two days before the sighting and had been resting and nursing at Coffs Harbour, Scott's Head, Port Macquarie, Forster, Port Stephens, and Swansea along the NSW north coast.

The whales are currently in waters northeast of Long Reef but were first sighted by whale watchers in Coffs Harbour on July 27

NSW wildlife rangers are working with ORRCA, marine rescue, police, and transport authorities to monitor the whales and ensure they remain undisturbed by the public.

Southern right whales are an endangered species, with fewer than 270 in the south-east Australian population.

This calf is one of only two recorded in NSW this year.