Five shots were fired at the offices of the CFMEU (Construction, Forestry, Mining and Energy Union) in Melbourne’s CBD early Wednesday morning.

Witnesses said up to five shots were fired into the Swanston Street building about 3am.

The bullets blasted through the front door, breaking a large glass window and causing significant damage.

A witness, who did not want to be identified, said the gunman fired the shots before driving off.

“I heard a bang. A male [was] holding a long gun. I saw it’s a hunting gun,” he said.

“He shot like five times or something, [then] the driver just drove away.

“I was on the balcony. It was very loud.”

At this stage the motive for the shooting is unknown but CFMEU secretary John Setka said the list of suspects would be a long one.

“Our priority is to look after staff at the office who are shocked and frightened,” the union said in a statement.

The shooting caused significant damage to the building. Photo: AAP.

“Thankfully no-one was harmed and we can confirm that all of our staff are safe.”

Victoria Police Detective Sergeant Haydn Beale said they were reviewing CCTV footage in a bid to identify the gunman.

“The fact that someone’s determined to get out of a car and let off a firearm at a building is a pretty concerning and a dangerous thing to occur,” he said.

“The fact that it has happened in the city obviously where there could be a lot of people going to work is again a major concern as well.”

No one was injured. The gunman remains at large.