A man has apologised for starting a large grassfire north of Melbourne as fire crews constructed control lines ahead of potentially dangerous conditions.

The blaze at Flowerdale, about 90 minutes from the CBD, has increased to 800 hectares since it started burning on Tuesday.

No homes have been lost and it’s understood no other structures like sheds or fences have been damaged.

Local resident Bill Sangster, 71, on Thursday took full responsibility for the blaze which he said was a “bloody accident”.

Mr Sangster said the fire started when an angle grinder he was using on a fence shot sparks into the dry grass.

“The waxy stuff on the tussocks exploded and spread, then the wind came up,” he told TV crews.

“Then it just took off.”

He told media it was not a fire ban day and he was not an arsonist.

Police on Thursday confirmed it wasn’t a deliberately lit fire.

Mr Sangster was arrested and questioned by investigators probing the cause of the fire on Wednesday.

More than 100 firefighters were at the fireground on Thursday, doing back-burning works and creating 17 kilometres of control lines.

Residents are being warned smoke will be visible for days all the way to Glenburn, Kinglake and Whittlesea as firefighters focus on the southeastern section of the fire.

The area will be under a high fire danger rating on Friday, with forecast strong northerly winds and temperatures in the low 30s.

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On the other side of the state, a total fire ban has been declared for the Wimmera and southwest regions.

Residents in those areas will not be allowed to light any fire in the open air until 11.59pm on Friday.

“Tomorrow’s conditions could make it difficult for firefighters to suppress a fire should one start,” CFA chief officer Jason Heffernan said.

“There is heightened fire activity across the state … and we’re asking the community to do its part in preventing further fires from starting.”

Heatwave to peak

The weather bureau issued warnings for heatwave conditions in Victoria and South Australia on Friday.

Victoria’s severe heatwave is expected to peak Friday with maximum temperatures in the mid to high 30s, impacting Heywood, Port Fairy, Portland and Warrnambool.

A cold front coming through on the weekend is due to bring welcome relief.

South Australia faces extreme fire dangers thanks to very hot, dry and windy conditions.

Residents are on alert in Eastern Eyre Peninsula, Lower Eyre Peninsula, Flinders, Mid North, Mount Lofty Ranges, Yorke Peninsula, Upper South East and Lower South East

A cooler change will move across western and southern parts of the state during the day, bringing possible thunderstorms.

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