A fire burning out of control north of Melbourne has threatened properties, prompting authorities to issue an emergency warning for residents at Flowerdale and Yea.

More than 41 fire trucks and six aircraft are fighting the grassfire, which is burning at Spring Valley Road in Flowerdale and travelling in a northern direction.

Residents have been urged to take shelter indoors immediately as it is now too late to leave.

Country Fire Authority State Duty Officer Neville Collins told ABC Radio Melbourne the fire has already covered 180 hectares and was still developing rapidly.

"There were a few houses under threat so there was a leave now that was put in place, or if they didn't get out they needed to shelter in place," he said.

The CFA has advised residents sheltering indoors to close all outside doors or windows and keep sight of where the fire is outside.

Those who are unable to get indoors have been advised to take shelter in the middle of a large open area such as a paddock or football oval, or get into a large body of water, such as a dam or lake.

Those in vehicles have been told not to travel to the area.

The fire activity comes amid an increased risk of grassfires for those living on Melbourne's ever-expanding suburban fringe after months of wet weather caused huge amounts of grass growth.

The 2022 spring was the state's wettest on record, and the lead-up to Victoria's summer was marked by floods across much of the state.

Last week, Melbourne saw its first 40-degree-Celsius day in three years.

Authorities have warned the wetter-than-usual spring means the fire season could last into March or even April.