A large grassfire is burning out of control north of Werribee, with the CFA issuing a 'watch and act' alert for surrounding areas including Tarneit and Truganina.

About 36 CFA and MFB trucks are battling the 80 hectare blaze at Shannahans, Dukelows and Dohertys Roads in Mount Cottrell, about 34 kilometres west of Melbourne's CBD. 

Firefighters were called to the scene about 5.30pm on Wednesday following reports of a large grassfire. 

The temperature in Melbourne was still hovering around 32 degrees at 8.30pm Wednesday, with mercury expected to hit 34 degrees on Thursday. 

There is a low-moderate fire danger rating in Melbourne. 

The fire is moving south towards Leakes Road and Cobbledicks Ford with the CFA waning residents in Chartwell, Eynesbury, Mount Cottrell, Tarneit and Truganina to be on high alert.

The CFA said the blaze was burning in an area bounded by Mount Cottrell, the Werribee River, Atkinson Road and an area between Leakes and Sayers Roads.

The fire authority is urging people who live next to grassland to move at least two streets away from the fire.

"If you live directly next to grassland quickly move two or more streets away from the fire," the CFA said. 

"Grass fires are unlikely to spread past the first row of houses. If your home is surrounded by grasslands, and you are not in an estate, you should immediately shelter within your home. It is not safe for you to leave." 

There is a significant amount of smoke in the area, if you suffer from exposure to smoke, seek medical advice or call Nurse on Call on 1300 606 024.