The New South Wales Department of Primary Industries (DPI) says restrictions on shark fishing are not to blame for several recent attacks around Sydney.

There were two shark attacks within two days in Sydney last week.

To ensure shark sustainability, fishermen in New South Wales are restricted to a catch of four sharks a day, after general shark fishing reached its 60 tonnes quota.

DPI chief scientist Professor Steve Kennelly says not all sharks are a threat.

"The sharks that are implicated in shark attacks on humans in New South Wales are usually, mostly, virtually all, bull sharks, tiger sharks and white pointer sharks," he said.

"Now those are caught extremely rarely in those fisheries, it's a different species of sharks that are caught in those fisheries.

"It's sharks that are not implicated in attacks on humans."

The Department says shark attacks are rare, but can be avoided by not swimming at dawn and dusk and swimming between the flags or in groups.

The DPI says shark numbers in Sydney waters have increased as a result of healthier ecosystems.

Professor Kennelly says sharks are a necessary part of New South Wales's environment.

"The ecosystem in Sydney Harbour's improved a lot over the last decade or so," he said.

"To the point where we're getting whales underneath the Sydney Harbour Bridge and we're seeing it return to quite a normal place, with lots of good bait fish coming in, tailor, king fish and so on and sharks feed on those things.

"So we're seeing that sort of ecosystem returning to normal."