Temperatures have halved in Victoria overnight but firefighters are still battling several fires across the state which are burning out of control.

Around 200 fires are burning in the state, six of which are a concern to firefighters, Two fires – in Rosedale and Strathbogie – are considered the most harmful and it is believed they may have been deliberately lit.

Melbourne is headed for a top of 21C on Saturday just a day after being belted with scorching heat, with some northern centres reaching 46C on Friday before the cool change swept through.

The milder conditions have brought relief to firefighters battling two major blazes to the north and east of Melbourne on Saturday morning.

Victorian SES state controller Tim Wiebusch said he believes if weather conditions remain favourable, firefighters could get the two main fires under control over the next three days.

But he warned flares up could occur at any time.

“Even sea breezes as you would expect in Rosedale will continue to be the challenge over the coming days. Whilst we have got benign fire conditions for the next 48 to 72 hours, that will help our fire crews and aircraft establish those control lines and blackout really well, but yes, as the next few days come along, even sea breezes will continue to be a challenge point for that fire.”

Authorities said 40 firefighters yesterday found themselves facing the fire head on after a sudden wind change. They survived the incident unscathed, but are shaken and are receiving support.

A watch and act alert is in place for residents near Rosedale, in the state’s east, which has been burning overnight.

Victoria Police Deputy Commissioner Andrew Crisp said the the cause of the fire is being investigated, but it is being deemed suspicious.

“Look, at this stage, nothing other than there were no lightening, dry lightening strikes in that area at this particular point in time. So, it seems unusual that these fires have actually started. So therefore, they’re deemed suspicious and we will have to let those investigations run their course.”

He urged members of the public to report any incidents of arson.

“On a day such as we had yesterday, or any day, where anyone intentionally lights a fire and puts people at risk of serious injury and death, it is abhorrent and that’s why it is so important that we encourage members of the public, if you see suspicious activity, that you report that. Whether that’s 000 or Crime Stoppers.”

Emergency services warned people in Kilmany, Fulham and Wurruk to monitor the dangerous conditions as the fire burns in a north-easterly direction towards them.

Meanwhile, a second watch and act is in place for a grassfire burning near Ancona, Merton and Strathbogie.

A wind change helped firefighters slow the spread of the fire overnight, but nearby residents have been warned to stay on high alert through the morning.

Victoria Police told AAP it was investigating the cause of the Rosedale fire.

Emergency Services Minister Lisa Neville warned on Friday that the “danger period is not over” with scope for more blazes.