A man in his 40s has died after he tumbled 18 metres from an aerial work platform at a school building site in Melbourne's southeast.

The 48-year-old man was operating a boom lift on a temporary track at a primary school in Wheelers Hill on Thursday afternoon when ground gave way on one side.

This caused the boom lift to tip and the man fell to the ground.

"Falls from height are one of the biggest workplace killers in Victoria," WorkSafe Victoria acting construction director Allan Beacom said in a statement.

"We know that the risk of injury from a fall increases significantly for falls above a height of two metres.

"Employers have to control risks from falls - if there's no alternative but to work at height, you've got to have effective fall protection or some other backup system in place."

Three Victorian men have died falling from a height at work so far this year.

Nine have died in similar circumstances since the start of 2009.