Environmental protection of Sydney's beaches and harbour has created a cleaner marine environment, but is attracting sharks closer to shore chasing fish, say marine experts, after two shark attacks in two days.

Fishermen say shark numbers are on the rise, but marine scientists say while there have been more sightings it is unclear whether there are more sharks off Sydney.

A shark almost severed a surfer's arm in an attack just before dark at Sydney's famous Bondi Beach on Thursday. The first attack at Bondi since 1929.

A navy diver was attacked in Sydney Harbour near the Opera House on Wednesday, losing his hand and he may lose his leg. New South Wales state Primary Industries Minister Ian MacDonald said on Friday he would order a survey of shark numbers off Sydney, after the attacks and as a result of more sightings.

"I think you'll find that the protective measures that have been put in place by governments in recent years have halted the decline of many species of sharks," MacDonald said on Friday.

"Coupled with some improved environment conditions, plus a reduction in fishing efforts in parts of the state, would mean shark numbers could enhance," MacDonald told local radio.

"The reports I am getting from people spotting sharks, there seems to be a build up in sharks in the estuaries, as well as along the ocean shore," he said.

PATROLS INCREASED, BONDI CLOSED

Many shark species, including the Great White, are protected in Australian waters. There are 30 sharks, including the Great White, on the International Union for Conservation of Nature's threatened species list.

Beach rescue helicopter patrols were increased along Sydney's beaches on Friday and Bondi Beach was closed for the morning, but some surfers still ventured out into the waves.

"Shark sightings have increased, particularly in the past five to six years," said Harry Mitchell who conducts aerial shark patrols over Sydney's beaches.

Mitchell said cleaner ocean waters had made it easier to spot sharks from the aerial patrol. "Shark sightings do not necessarily mean shark numbers have increased," he said.

Bondi Beach lifeguard Bruce Hopkins said sharks regularly swam into Bondi, despite shark nets aimed at protecting swimmers.

"We usually get all types of sharks come through Bondi. We get hammerheads and the grey nurse ... with the occassional tiger or bull shark. It's nothing out of the ordinary," he said.

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NSW National Parks and Wildlife Service said sharks were increasingly common in Sydney Harbour, particularly in February and March as they chase seasonal fish into the harbour.

"I guess it's the downside of the environmental controls ... 20 or 30 years ago the harbour was a very difficult place to be for a fish, these days it's actually quite beautiful," said the service's John Dengate.

In the past 10 years Sydney Harbour has become a much clearer marine environment due to a reduction in commercial shipping, less foreshore marine industry, a ban on waste discharge and boat paint containing lead, and better stormwater control measures.

Sydney's beaches have also become cleaner due to the installation of offshore sewage treatment plants and better stormwater controls. While large stretches of Sydney's coastline are now protected marine sanctuaries to encourage marine life.

Surf Life Saving Australia (SLSA), whose volunteers patrol beaches, called for calm on Friday.

"It is important to recognise that there is always some inherent risk when using an environment inhabited by sharks," said Barry Bruce, a scientist with the Commonwealth Scientific & Industrial Research Organisation and SLSA shark advisor.

"The risk of shark-related incidents varies according to the time of day, time of year, the geographic location and species of shark in the area," said Bruce.

The SLSA advises people not to swim at dawn and dusk, when sharks usually feed, or in known shark feeding areas, such as estuaries or harbours.

The last fatal attack occurred in December 2008, when a Great White attacked a 51-year-old man while he was snorkelling off a beach south of Perth in Western Australia.

There have only been a total of 56 fatal shark attacks in Australia in the past 50 years, or an average of about 1 a year, says the Australian Shark Attack File.

** For more, take a look at the feature:

"Sharks, not humans, most at risk in ocean