Police have praised the swift actions of two bystanders on Rottnest Island who helped rescue an 11-year-old boy, his mother and a female relative, who were injured when a jetty collapsed underneath them this morning.

Key points: Group was fishing on a jetty when the corner gave way

Group was fishing on a jetty when the corner gave way One woman was trapped between concrete slabs and the jetty

One woman was trapped between concrete slabs and the jetty All three have been airlifted back to Perth with head and leg injuries

It is understood the boy and two women, aged 63 and 48, were fishing off the Army Jetty, at the southern end of Thomson Bay, when the structure gave way about 6:20am.

The boy and his parents were on holiday in Perth from the UK. He suffered head injuries and the two women received lower leg injuries.

All three have been airlifted to hospital in Perth.

Sergeant Phil Cartledge, from Rottnest Island police station, said the three fell into the water when the jetty collapsed, trapping the older woman between concrete slabs and the jetty.

The mother, with the help of her partner, had to hold her son's head above water until help arrived.

The collapsed section of the jetty at Rottnest Island. ( ABC News )

Bystanders rushed to help

Sergeant Cartledge said the outcome of the accident could have been worse had it not been for a woman who witnessed the accident and rushed to help the family, and 80-year-old David Hawks, a sailor who was asleep on his yacht when the accident happened.

Mr Hawks paddled across in his dinghy to rescue the boy, who appeared to be unconscious, and took him back to the beach.

"He was moaning, he was obviously bleeding quite extensively," Mr Hawks said.

"His mother, who'd obviously been holding him and whom I didn't know to be injured, then swam or managed somehow or other to come from the jetty and literally crawled up the beach."

80-year-old David Hawks was asleep on his yacht at the time of the accident and rushed to the scene in his dinghy. ( ABC News: Nic Perpitch )

Meanwhile, Sergeant Cartledge dived underwater to help the older woman whose leg was trapped between concrete slabs and the jetty.

"She was in severe pain," he said.

"She would have been there for up to 40-50 minutes.

"The partner was in the water and they were both suffering from hypothermia at that stage, shaking heavily."

Rottnest infrastructure to be assessed

Water and Fisheries Minister Dave Kelly said there would be an investigation into the accident.

"It's very early at this point in time to know exactly what's happened so it's probably not helpful to speculate as to what the cause may have been at this stage," he said.

"Obviously there will be a thorough investigation to determine what the cause was and what needs to be done in the future."

The collapsed section of the jetty at Rottnest is fenced off. ( ABC News: Nicolas Perpitch )

Mr Kelly said the priority for now was making sure the family involved was receiving support.

"Our thoughts are with the family. We wish them a speedy recovery," he said.

"It was obviously a very traumatic experience for those involved but our priority is to make sure that they get the best possible care."

Mr Kelly would not respond to reports that the Rottnest Island Authority had been requesting funds to repair the structure.

WA Premier Mark McGowan said other infrastructure on the island should also be assessed following the accident.

"We clearly now need to look at that infrastructure, it has been there for a long period of time," Mr McGowan said.

"I'm sure the Rottnest Island Authority will now do a review of the jetties and other infrastructure to see if there's any further issues."