Staff have returned to Melbourne Airport's air traffic control tower, after a fire alarm halted plane departures and arrivals.

Flights in and out of Melbourne Airport were delayed after the alarm went off in the tower just after 9am.

Melbourne Airport spokesman Grant Smith told 3AW the tower, operated by Airservices Australia, had to be evacuated.

Departures and arrivals were halted, with planes forced to circle in a holding pattern until being cleared to land.

A Qantas spokesman said it caused two Melbourne-bound flights to be diverted to Canberra, resulting in about a 30-minute delay, but they were now returning to Melbourne.

Air traffic controllers have since been given the all-clear and have returned to their desks.

An Airservices Australia spokeswoman said there was no fire and what sparked the alarm at 9.12am was unknown.

Staff returned to the tower about 40 minutes later.