The highway was closed and people were being asked to avoid the area.
A helicopter unloads a monsoon bucket on a scrub fire near Tikokino. Photo / Warren Buckland Area Commander Ken Cooper said the fire covered an area of grass and forestry land.
"We've had to close the road for people's safety, and to allow us to get helicopters into the area to fight the blaze," he said.
"Anyone travelling in the area will need to take an alternate route until we can safely re-open the road."
TikokinoFireFire and Emergency said State Highway 50 was likely to remain closed until at least lunchtime.
"People are asked to avoid the area while emergency services work to put out the fire."
Newstalk ZB caller John saw emergency services descend on the scene shortly before 11pm.
"Something major's going on. There's been 10 or more engines going past, tankers, you name it.
"It's undulating farmland, some big homesteads and woolsheds out that way - and obviously it's a big one."