Bronzewing goes around a pair of 'mating' Southern Right whales in the channel between Garden Island and Pinchgut. Photo by SGT Bob O'Donahoo.

What does a tugboat do when she comes across a pair of whales making love in the channel?

She throttles back, turns hard to starboard and gives them a wide 'berth'.

Such was the case for the master and crew of the Defence Maritime Services' RAN support tug Bronzewing on Sydney Harbour the other day.

The tug was on her way from the Garden Island boat pound to Fleet Base East when she encountered the pair of 10-metre long Southern Right whales in the channel between the end of the island and Pinchgut.

The whales, along with a third, entered the Harbour the previous day.

Experts suggest one was a female in estrous, the others male suitors.

Off Watson's Bay she spurned one male and he went back to sea.

With the other she gambolled west up under the Harbour Bridge and as far as Blues Point.

The sight of the whales attracted thousands of people to harbourside vantage points. Others hired water taxis to gain a closer look.

Waterways Authority and National Parks and Wildlife Service officers were in attendance and made sure all craft were keep outside the mandatory protection circle. Sydney ferries reduced speed and passengers lined their decks.

The 'loving' pair spent a day in the Harbour before heading to the open ocean.

Their appearance so far into the Harbour was seen as "extremely rare".

Whale sightings off the Sydney coast are however now a common occurrence as the ban on whaling by most countries allows for the mammals to increase in numbers.

Southern Right whales migrate annually from the Southern Ocean and travel as far north as Queensland.

By Graham Davis