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 risking losing 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," Mr 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," Mr MacDonald told local radio.

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

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 Sydneys 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.

Mr 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 occasional tiger or bull shark. It's nothing out of the ordinary," he said.

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, 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, geographic location and species of shark in the area," said Dr 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 last December, 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 one a year, says the Australian Shark Attack File.

How to avoid, or survive, a shark attack

• Don't swim in a known shark feeding area, such as the estuary of a river, creek or a nearby beach or harbour.

• Don't swim at dawn or dusk, when sharks normally feed. Don't swim among schools of fish, particularly swarming bait fish which are a favourite of sharks, or a colony of seals.

• If you do find yourself in the water with a shark, its best to get out as quickly, but as quietly as you can. Don't panic. Panicking and thrashing the water will just make the shark think you are a school of fish.

• If exiting the water is not an option, then crowd together or at least try and make yourself look big. You may intimidate the shark and it might back off. Remember this is the food chain, so size and aggression often win.

• Don't give up, fight !!! Three of the Australians bitten since January survived because they or their rescuer punched the shark in the head. Sharks are sensitive around the eyes, gills and nose and divers who routinely swim with sharks use small batons to prod off sharks if they get too close.

• Even if you find yourself inside the shark, keep on fighting! An Australian abalone diver was swallowed head first up to his chest by a Great White in 2007, but kept punching and poking the shark in the eye until it let him go.

He survived because he was wearing a lead vest which prevented the shark's bite penetrating his chest and because his punching forced the shark to release its bite.