Taupō fire crews spent six hours fighting a grass fire in Ohaaki, halfway along State Highway 5 to Rotorua, last night.

Fire and Emergency New Zealand shift manager Colin Underdown said an underground fire at the former rubbish dumping site had been burning slowly for years, but dry grass ignited on the surface of the skip in yesterday's heat.

"It was 100m by 300m in size, and there was one tanker, one pump and several rural fire crews at the scene."

He said the site was between the State Highway 5 intersections with Dan Rd and Horouta Rd.

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Crews were first called just before 6pm and didn't leave until midnight.

Underwood said he was not yet aware of any follow-up work at the site today .