About three-quarters of Dunedin's main water supply could be out of action for up to a year after a massive blaze engulfed an area near Middlemarch at the weekend.

Voluntary water restrictions will likely remain in place over summer after fire burned across 4664ha at Te Papanui Conservation Park on Old Dunstan Rd.

90 firefighters will be back at the scene today along with infrared cameras to tackle hot spots, as an investigation into its cause begins.

The council stopped taking water from the catchment on Saturday, but the water is now feeding into the city's water supply.

Mayor Aaron Hawkins told Andrew Dickens Dunedin residents should think twice about watering the garden, leaving the sprinkler on, or washing the car.

"We'll monitor water use and weather, which is another key factor over the summer, and if we had to we would move to more formal restrictions."

Hawkins said the fire was a significant event, but would have been more severe without the efforts of firefighters.

Dunedin City Council 3 Waters group manager Tom Dyer said the Deep Creek Reservoir would become Dunedin's primary water source as a result of the fire.

Deep Stream would be out of action for anywhere from three months to a year, he said.

Dunedin's average water use was about 44,000 cubic metres a day, he said.

This would continue to be met from a range of treated water sources.

At the briefing yesterday afternoon, Otago district principal rural firefighter Graeme Still said the fire had been contained, but hot spots existed.

At its peak on Saturday, 11 helicopters with monsoon buckets worked to extinguish the blaze above four ground crews.

Fifteen millimetres of rain on Saturday night helped firefighting efforts.

It marked the third major vegetation fire in Otago in the past week.

A fire at Cornish Pt, near Cromwell, last Monday covered 8ha.

The cause was still under investigation.

A fire at Bucklands Crossing near Waikouaiti, which covered 16ha and was believed to have been caused by a vegetation burn-off reigniting, was extinguished yesterday.

A fire investigator would begin work today to establish the cause of the Middlemarch fire.

Fire crews were able to respond quickly because they had previously planned for a potential fire in the area, he said.

About 1100ha of the fire was on Department of Conservation land.

Winds of up to 100km/h, warm temperatures and no vehicle access to the area meant serious challenges for firefighters.

At its peak the fire was spreading across more than 1.5km of land per hour.

A normal rate would be about 300m an hour, Still said. 

- text by Otago Daily Times