Speaking outside, Chief Inspector Tony Townsend said it was a "very tragic incident" on what had been a family day out.

"[It's] very distressing for everyone involved," Chief Inspector Townsend said.

Nine News reported that the incident unfolded in front of a group of relatives and one woman collapsed in shock.

Inspector Jamie Warhurst from NSW Ambulance said the incident was "heartbreaking".

"No words can describe the scene we were faced with this afternoon," Inspector Warhurst said.

"The lifeguards, bystanders and police did an amazing job, providing resuscitation until paramedics arrived. They should be commended."

A NSW Police spokesperson said officers from Port Stephens-Hunter Police District "attended and have commenced inquiries".

"A report will be provided for the information of the Coroner," the spokesperson said.