A large scrub fire near the North Island's Desert Road was sparked during a Defence Force live fire training exercise.

Fire crews responded to the blaze on the Central Plateau about 10.15am on Monday, Fire and Emergency New Zealand (Fenz) shift manager Becca Jennings said, and it appeared to be a continuation of a fire from the previous day, which had escalated.

By mid-afternoon on Monday the flames covered an area about 400x400 metres, but it had spread to about 1.5ksq m (238 hectares) of vegetation by evening.

Five helicopter crews and nine firefighting crews were working on the blaze.

The New Zealand Defence Force said the fire, which was in their military training area, was sparked by gunfire.

"The fire initially started on Sunday, during live fire training, and was thought to be controlled and extinguished, before it reignited on Monday morning," they said. "The blaze may take a further 24 hours to extinguish."

Thirteen defence force staff are battling the blaze alongside Fenz efforts.

The fire was in a fairly isolated area. There had been no reports of injuries and it did not pose a threat to the Desert Road or to buildings in the area, Jennings said.

Motorists travelling on the Desert Road - State Highway 1 - captured footage of large plumes of smoke just south and north of the Waiouru township.