Multiple people injured in bus crash, one critically, in Perth's north

A bus crash in Wanneroo has injured several people, at least one critically. (ABC News: Mya Kordic)

One person suffered critical injuries when a bus collided with a tree on Joondalup Drive in Wanneroo in Perth's north.

Another six passengers were taken to hospital after being hurt in the crash.

The accident came after a car collided with a pedestrian before crashing into a hospital in Perth's south, forcing the closure of its emergency department.

A woman suffered critical injuries and six other passengers were hurt when a Transperth bus crashed into a tree in Perth's north on Thursday morning.

Police say the bus was travelling west along Joondalup Drive just after 11am when "for unknown reasons, the driver lost control and collided with a large tree".

Multiple people onboard the bus were injured, including a 59-year-old woman who was taken to Royal Perth Hospital with critical injuries.

At least five people were injured when a bus crashed into a tree in Wanneroo. (ABC News: Mya Kordic)

A 61-year-old male passenger was also taken to Joondalup Health Campus with serious injuries.

The 56-year-old male bus driver was taken to Royal Perth Hospital for medical treatment.

The accident caused heavy traffic in the area. (ABC News: Mya Kordic)

Pictures show the front of the bus was significantly damaged after smashing into the tree.

Police remained at the scene on Thursday afternoon, with forensics officers marking up Joondalup Drive as they try to piece together what happened.

Police are trying to find out how the accident happened. (ABC News: Mya Kordic )

The crash forced the closure of Joondalup Drive between Burns Beach Road and Wanneroo Road, with motorists asked to avoid the area.

Tia Anderson said she was driving when she saw the bus swerve into the opposite side of the road and then crash into the tree.

Tia Anderson was driving when she saw the bus crash. (ABC News: Michael Tetlow)

"Suddenly it went into the [median] strip into the opposite lane and it was just crazy," she said.

"...it was really horrific because there was a lot of ... people terrified, people on the scene and someone in really critical condition."

Ms Anderson said she was one of the first people at the scene, and was quickly relieved by emergency services.

The incident happened about an hour after a car crashed into St John of God hospital in Murdoch, in Perth's south, forcing the closure of its emergency department.

A car crashed into a treatment ward at St John of God Murdoch Hospital on Thursday morning. (ABC News: Cason Ho)

Police say a grey Nissan Tiida sedan w