High-rise Melbourne construction sites can only keep functioning with one-quarter of their normal employees on-site under sweeping new rules announced on Monday to curb the spread of the pandemic that threatens to sweep across the country's second-largest city.

From 11.59pm on Friday any construction project higher than three storeys, such as apartment towers and office buildings, will have to function with a "practical minimum" of staff but no more than 25 per cent of the total workforce for the next six weeks, Premier Daniel Andrews said.