A five-year effort to measure the exact dimension of greater Sydney Harbour has revealed that the estuary has nearly 80 km more shoreline and is 1.6 metres deeper than believed earlier.

Dennis Buttigieg, manager of the survey undertaken by the New South Wales Maritime Authority, said the new reading of the harbour was the first since the 1960s when most calculations were done manually, according to The Sydney Morning Herald.

 "They didn't have computers, so somebody would have sat down...and just worked it out," he said.

The best estimate of the estuary's volume at high tide had been about 500,000 mega litres. But the authority now says it is 562,000 mega litres.

The perimeter of the estuary was thought to be 240 km. Now it is known to be 317 km, thanks to new equipment that allows mapping to an accuracy of 20 cm or better.

With 160,000 digital soundings and latest computer software, Buttigieg and his team have found an average depth of 10.7 metres, compared to the previous 9.1 metres.

In the 1960s, the surface area of the estuary was calculated as 5,500 hectares. But it is now known that at mean high water mark, excluding islands, the area is 5,255 hectares.