In short:

Shark and public policy expert Chris Pepin-Neff says more communication is needed about sharks in Sydney Harbour after heavy rain.

There have been four shark-related incidents off the NSW coastline in 48 hours.

Surf Life Saving NSW CEO Steve Pearce all beaches on the northern beaches will be closed for the next 48 hours due to conditions.

"After 20 millimetres of rain, the level of faecal matter and raw sewage that are in the harbour attracts bait fish and brings sharks toward the bait fish," they said.

"It's really important after a big storm to not swim in the harbour for about 72 hours."