Firefighters are at last getting on top of a major blaze burning in Victoria's southwest as more hot weather arrives this week.

While temperatures remained high in the north of the state on Sunday, cooler temperatures in the south have helped authorities contain blazes.

Firefighters battling the huge blaze at Kentbruck, in the state's southwest, are getting it under control as it continues to issue massive plumes of smoke.

The fire, which began in a pine plantation, has burned over 2700 hectares and smoke haze is visible throughout western Victoria.

Motorists have been warned that thick smoke may restrict visibility along the Princes Highway between Greenwald and the South Australian border, with drivers are asked to reduce speed accordingly.

Fire Services Commissioner Craig Lapsley said crews and aircraft worked on the Kentbruck fire on Sunday, with the aim to get it under control before the weather heats up again.

"Tuesday is the day that has the northerly strong winds and again Friday. They are the two critical days, but we could still have fires on other days that could still do damage," he told reporters in Melbourne.

"It is a week that people need to stay vigilant about fires in Victoria."

On Sunday evening, the water bombing helicopter Elvis helped bring a grass fire under control at Little River, near Geelong.

Mr Lapsley urged people to dob in arsonists.

His comments came as an 18-year-old woman was charged with deliberately starting a fire in Melbourne on Saturday.

She was arrested at the scene of the fire, which was burning on an embankment near Warrigal Road at Ashwood about 9.45pm (AEDT) on Saturday.

Emergency crews quickly extinguished the blaze.

The woman has been charged with intentionally causing a bushfire and recklessly endangering life and was bailed to appear in Dandenong Magistrates Court on April 29.

Mr Lapsley said the Country Fire Authority (CFA) will continue to monitor the Fireready app and CFA website on Monday to ensure people have access to information they need.

Many people had trouble accessing the site during hot weather on Friday.

The site will again be tested with temperatures set to be above 40c in the north of the state on Monday, while Melbourne is expecting 32C.

North-eastern parts of the state are already sweltering, with the towns of Wangaratta, Yarrawonga and Rutherglen reaching 42C on Sunday.

A fire burning near Ensay, in east Gippsland, was brought under control on Saturday afternoon.