Shark-spotting helicopter patrols will fly over Sydney beaches in the summer months of December and January following reports of a rise in the predators' numbers off Australia's biggest city.

Sydney has seen three shark attacks this year, two on surfers at Bondi and Avalon Beach and one on a navy diver in Sydney Harbour. All three people survived the attacks.

For decades, Sydney's beaches have had shark nets stretched offshore to prevent attacks, but authorities said on Wednesday a trial of aerial shark-spotting patrols would be staged between December 19, 2009 and January 10, 2010.

"These days where there are very large numbers of bathers on our beaches we want to add to the current programme, the meshing programme, in protecting people who swim off our shore," said New South Wales state Primary Industries Minister Ian Macdonald.

"However, there will never be 100 percent guarantees when swimming in our oceans," he told reporters.

Environmental protection of Sydney's beaches and harbour has created a cleaner seas, but it is also attracting sharks closer to shore as they chase fish, marine experts say.

Many shark species, including the Great White, are protected in Australian waters.