Two hospitalised as tornado rips through Bunbury, ripping off roofs ...

Two people have been rushed to hospital after a tornado smashed through the city of Bunbury, in WA's south west, on Friday afternoon, ripping off roofs and damaging property.

St John Ambulance said a woman in her 30s who was at the Bunbury Police and Citizens Youth Club, which was in the path of the storm, sustained non-life threatening injuries and was taken to hospital.

The Bunbury PCYC sustained significant damage in a tornado that affected parts of WA's south west. (ABC News: Ethan French)

It's understood a man was also taken to hospital from the Bunbury Regional Prison with less serious injuries.

Pictures from the scene showed a large part of the PCYC building and its roof collapsed, with huge twisted sheets of metal extending into the car park.

A spokeswoman for St John WA said six ambulances were called to the PCYC.

South Bunbury resident Luke Clark said the children of some of his friends had been doing gymnastics at the PCYC when the tornado hit.

"I hope they are all going good, all the kids," Mr Clark said.

Two people have been taken to hospital in the aftermath of the storm.(ABC News: Ethan French)

While there were severe thunderstorms forecast for the area, the tornado took residents by surprise.

"You see the cold fronts and you know there is going to be rain but just the force of it, and obviously the damage that it's caused yes it was a bit surprising," Mr Clark said.

He and others had turned out to help clean up the damage at the Bunbury Crochet Club, near the PCYC.

"It's looking all good now," he said.

"We have just reinstated the fence and secured it as much as we can."

Department of Fire and Emergency Services (DFES) South West Superintendent Andy Wright said it appeared the tornado's impact was localised to the Withers suburb of Bunbury.

"Forty roofs have been impacted, from small amounts of damage, to being lifted off," he said.

A man has been taken to hospital from Bunbury Regional Prison.(ABC News: Jacqueline Lynch)

"The early reports of injuries were worrying but at this point the only injuries are quite superficial, and most people appear safe."

Mr Wright said the severity of the storm system had been unexpected.

Large parts of the Bunbury PCYC building were reduced to giant sheets of twisted metal when the tornado tore through. (ABC News: Ethan French)

"The surprise of it all is what people are dealing with and [they] will be taking stock of the damage to their properties," he said.

The Department of Fire and Emergency Services confirmed a tornado had been reported in South Bunbury around 4pm on Friday.

The Bureau of Meteorology subsequently issued an updated severe weather warning for WA's South West and Great Southern districts at about 4:27pm.

Bunbury regional prison suffered significant damage in the tornado.(ABC News: Jacqueline Lynch)

The bureau's Luke Huntington said assessments of videos posted to social media contributed to the tornado's classification.

"They are fairly common in this time of year and into the winter months… on average five to six reports of tornadoes per season," he said.

Bunbury PCYC has sustained significant damage in the tornado.(Supplied)

Mr Huntington said thunderstorms were likely to produce damaging winds and heavy rainfall that may lead to flash flooding.

Areas including Bridgetown, Bunbury, Collie, Manjimup, Harvey and Walpole are expected to be affected.

People in the area are being urged to be aware of fallen powerlines, trees and other hazards.