Three men have been killed in two separate workplace accidents in Perth, including two who were crushed to death after a concrete panel fell on them in East Perth.

The third man is believed to have died after falling from scaffolding at an aluminium refinery in Kwinana this afternoon.

Department of Mines and Petroleum State Mining Engineer Andrew Chaplyn said the scaffolder was believed to have fallen 10 metres while working at the Alcoa refinery.

"Inspectors from the department will be onsite this afternoon to investigate the incident," he said.

"The department offers its sincere condolences to the worker's family, friends and colleagues."

The East Perth accident happened on a Jaxon apartment building site near the corner of Bennett and Goderich Streets about 11:35am.

Police at the site told the ABC that the two workers suffered fatal injuries when a construction slab fell on them.

The ABC understands that one of those killed was from Ireland.

One witness, Travis, said the slab appeared to have fallen off a delivery truck while it was being unloaded.

He said he tried to help lift the panel off one of the men.

"All I was thinking of was to just get the slab off him, so a whole bunch of workers jumped on one side of the slab, because it was sort of sitting on another slab, like a concrete barrier.

"A whole bunch of guys jumped on that side while a few of us lifted the other side ... so it was a couple of inches off them."

He said he backed off when emergency services arrived.

Police, St John Ambulance and the Department of Fire and Emergency Services attended the scene and the area was cordoned off.

Worksafe, union probe accident

An investigation into the East Perth accident is underway.

"Due to the incident being at a construction site, WorkSafe have been advised and they are attending the scene to investigate the circumstances surrounding the incident," a police spokesman said.

"WA Police will prepare a report for the coroner."

CFMEU assistant secretary Joe McDonald said the union would carry out a full investigation of its own to determine what happened at the Jaxon construction site.

He said it was "absolutely terrible" that two young men were "no longer with us".

"I am not going to make any comment on the accident until there has been a full investigation, other than two workers have lost their lives," he said.

"[They] went to work today and they're not going home."

All Jaxon sites to close tomorrow

Jaxon said the apartment site remained closed and was under the control of the WA coroner.

Emergency services were unable to save the two men at the East Perth building site. ( ABC News: Laura Gartry )

"Our heartfelt thoughts and condolences go out to the families, loved ones and colleagues of the two men involved in this tragic accident," the company said in a statement.

"Counselling support has been offered to all 52 employees and contractors who were on site today and also to Jaxon's wider workforce."

The company said it had never before experienced a workplace fatality.

"Our workforce is grieving and our focus is on supporting the families and those affected. As a mark of respect, and to fully support our employees, all Jaxon sites will be closed tomorrow," the statement said.

Mr McDonald said the union held concerns for safety standards at Jaxon worksites.

However Jaxon said safety was the company's highest priority, and said it was cooperating with the coroner's office and WorkSafe.

"It would be premature to speculate or pre-empt the findings of the investigation," the company said.

Police said the men were yet to be formally identified.

Including today's deaths, 16 people have died at their WA workplaces since July according to Worksafe.

In the last financial year the state recorded 22 workplace deaths, the highest in seven years.