Authorities will investigate the cause of a balcony collapse in which 19 people were injured during a Halloween party in Perth.

Thirty people were believed to have been on the balcony when the accident occurred, about 8pm (WST) on Saturday at a house on Bermuda Drive in the northeastern suburb of Ballajura.

Nineteen people were rushed to three different hospitals suffering broken bones, cuts and bruises.

A Royal Perth Hospital spokeswoman confirmed at least one person had a spinal injury.

Two people were discharged on Sunday morning with the remaining 17 in a stable condition.

It comes less than a week after a balcony collapse in Queensland, where eight people were injured after they fell three metres onto concrete during a wake at Morayfield, north of Brisbane.

West Australian police Inspector Trevor Davis said local government authorities from the City of Swan were investigating the cause of the collapse.

About 40 guests were at the Halloween party, aged between 59 and 77.

Insp Davies warned residents not to overcrowd balconies and check their house plan for how much weight their balcony was designed to hold.

"People must remember most balconies are designed in reality for a family group," Insp Davies said.

"In every instance I'm aware of, when there's been a balcony collapse, there's been in excess of 20 people on it."

The ABC has been told some are believed to have serious spinal injuries.

A neighbour said she heard a crash and screams when the balcony at the back of the property collapsed.

She said her house shook.

Police, St John's ambulance paramedics, Fire and Emergency Services Authority and State Emergency Services attended the scene.

The injured guests are being treated at Royal Perth Hospital, Sir Charles Gairdner Hospital and Joondalup Health Campus.