Police in Melbourne say four men have been injured, two seriously, after a cement truck cleaning pump exploded, spraying them with cement.

Key points: The incident happened at a construction site at 108 Southbank Boulevard

The incident happened at a construction site at 108 Southbank Boulevard Four men were injured when sprayed by high pressured concrete and aggregate

Four men were injured when sprayed by high pressured concrete and aggregate The force of the blast broke the window of a passing car

The incident happened about 1:15pm on the construction site at 108 Southbank, a new high-rise which, when finished, will be one of Melbourne's tallest buildings.

MFB Commander Rick Gili said the men were sprayed with crushed rocks, gravel and wet cement.

"The cleaning mechanism at the back of a concrete truck became dislodged and shot out debris that included bits of concrete and aggregate," he said.

"That aggregate hit three people who were working on site — one in the face, one in the back, and one in the back of the head."

The truck was delivering cement to the high-rise work site at 108 Southbank. ( ABC News: Dylan Anderson )

The force of the blast broke the window of a passing car.

Police say two men were taken to the Alfred Hospital in a serious condition and two others are in a stable condition at the Royal Melbourne Hospital.

One person in a passing car was cut by broken glass. ( ABC News: Dylan Anderson )

Commander Gili said about a wheelbarrow load of cement spewed out.

"It would've been enough to frighten a lot of people because it came out at very, very high pressure with a loud bang," he said.

Worksafe is investigating.