A boy and two women have suffered serious injuries and flown to Perth after part of the Army Jetty on Rottnest Island collapsed, trapping one person.

An 11-year-old boy has suffered fractures and head injuries, while his mother suffered leg fractures, as well as cuts and bruises.

He is in a stable condition in Perth Children’s Hospital.

A 48-year-old woman and a 63-year-old woman are both being treated at Royal Perth Hospital for fractures to their legs. They are both in stable conditions.

One of the women spent more than an hour in the water before she was rescued.

Camera Icon The jetty that collapsed. Credit: Seven News

DFES crews needed to use tools to help free the trapped person.

The trio have been flown to Perth for treatment.

It is not yet known if anyone else was treated at the scene by the island nurse.

The jetty collapse occurred at about 6.22am on Wednesday, while three people were fishing from the jetty.

One person needed to be rescued after their foot became trapped, while two others made it safely to shore.

Camera Icon The RAC Rescue Helicopter was deployed to fly the injured to hospital. Credit: Seven News

A Rottnest Island Authority spokeswoman said rangers had closed the area to the public but a structural assessment had not yet begun.

“Obviously there will be some follow up,” she told AAP.

“Our main priority is keeping people safe and away from the area.”

Fisheries minister Dave Kelly said there would be an investigation, likely conducted by the Department of Transport.

“There will be a thorough investigation to determine what the cause was and what needs to be done in the future,” he told reporters.

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

A 2014 report estimated it would cost about $3 million to replace the Army Jetty, which is largely comprised of limestone boulders and concrete.

The report found the Main, Army and Stark jetties had “outstanding and ongoing maintenance issues, which need to be addressed to ensure their ongoing structural integrity, requiring substantial investment”.

In the RIA’s 2017-18 annual report, the budget for repairs and maintenance on the island, excluding a roofing project, had been cut by $536,000 compared to the previous year.

Mr Kelly would not comment on suggestions the RIA had requested funding to fix the Army Jetty, saying it was not within his responsibilities.

Built in 1906, the 100-metre jetty was initially named the Excursionist Jetty and was used as a terminal for passengers arriving and leaving the island.

From 1935, it was upgraded for military purposes leading up to and during World War Two.

It was used for ferry berthing before the Main Jetty was built in the 1960s and is now used for fishing.