The boy, 17, died in John Hunter Hospital after he was apparently restrained by bystanders following an alleged assault on an employee of the swimming centre.

A parent told the Newcastle Herald the pool was in lockdown for several hours.

Police were called to the Lane 4 Aquatics centre on Durham Road swim centre shortly after 5pm, responding to reports an employee of the pool had been assaulted.

According to a statement, witnesses restrained the 17-year-old before police arrived at the scene.

The boy was treated at the scene by paramedics before being taken to hospital.

He died in hospital, police said.

The extent of the boy's injuries were not known with police refusing to comment any further on Tuesday night.

However, a parent told the Herald someone jumped in the pool and disrupted swimming lessons.

"Obviously people were coming from everywhere trying to help," the parent said.

"I went to pick up the kids and the pool was all in lockdown. We had to wait while they took the kids out the back of the pool."

Authorities launched a critical incident investigation, which is standard when a member of the public dies in the course of a police operation.

In a post on its Facebook page on Tuesday night, Lambton pool said it would be closed indefinitely.

"Due to unforeseen circumstances the Lambton Swimming Pool will be closed for an undetermined short period of time," the post said.

"We appreciate the patience and apologise for any inconvenience caused.

"We will update everyone as soon as possible.

Contact Crime Stoppers on 1800 333 000 or nsw.crimestoppers.com.au/

Newcastle Herald with Andrew Taylor