Firefighters are "close to containing" a large vegetation fire in Canterbury, as winds are forecast to strengthen.

Seven crews will be monitoring the blaze overnight, Fire and Emergency New Zealand said today in a 7.20pm update.

"It's important we keep on top of the fire tonight as we are close to containing it," incident commander Brian Keown said.

"We hope to have containment completed tomorrow before the extreme fire danger day on Sunday."

The fire covers around 980 hectares — almost double the size of Fire and Emergency's previous estimate yesterday.

Keown said Saturday will see an increase in ground crews and fewer aerial support, with five helicopters and no fixed-wing aircraft.

"There will be more work to do on the ground tomorrow than from the air," he explained.

"All structures in the fire ground have retardant lines laid around the properties so at this stage we do no require the fixed-wing aircraft to be operating, but they are still available should we need them."

Keown thanked firefighters and support staff for their efforts, and for locals and visitors for their patience while State Highway 73 was closed.

Crews returned to the fire this morning, after maintaining "active firefighting throughout the night".

The fire caused the voluntary evacuation of some houses in Castle Hill Village yesterday.

SH73 was closed overnight but reopened around midday with stop-go signs in place and traffic being escorted in convoys.

NZTA said motorists should expect "inevitable delays" due to the length of highway involved.

"The escorted convoys are necessary while Fire and Emergency have choppers operating over this area to put out the fires."

District commander Dave Stackhouse said the perimeter of the fire was now 17km long but that the weather was improving firefighting conditions.

The fire was now in what he called "benign" conditions despite not being controlled.

"We can't rest on our laurels, however. If the wind conditions come up, we need to increase our resources — that restricts our air attack options."

He said he was confident most of the heavy fuel, like dead standing trees, had been burned and extinguished.

"As long as the weather conditions hold, we'll get onto it quite well."

Stackhouse said one structure had been lost in the fire — an old New Zealand Forestry Service building called Holloway Lodge.

An update at around 3.30pm said Fire and Emergency had made "good progress" in containing and controlling the 980 hectare vegetation fire.

Incident Commander Brian Keown said weather conditions this morning supported fire suppression.

"We plan to hit the fire ground hard with ground crews and air attack this weekend to get as much of the fire controlled and extinguished before stronger winds forecast early next week arrive."

Ten helicopters, two fixed wing aircraft and 86 firefighters supported by heavy machinery are on the fire ground this afternoon.

Three fire investigators are on the fireground in a joint investigation with police that would likely take most of next week due to the scale of the blaze.

Keown said yesterday that it was possible, though not probable, the large vegetation fire at Bridge Hill would reach the village overnight.

But this morning, FENZ confirmed the fire hadn't entered the village.

"No emergency evacuations were needed, and the fire is still a safe distance from the settlement," an update at about 7.15am this morning said.

Keown said weather conditions today were more favourable for fighting the fire.

Crews "had a pretty good night", he told Breakfast this morning.

"We don't anticipate any further evacuations today, the evacuations yesterday were voluntary.

"Hopefully, by the end of the day, we'll have the fire pretty much contained."