Several fires that merged into one blaze closed the main trunk line near Waiouru yesterday.

Fire and Emergency NZ (Fenz) was alerted to the fires near State Highway 1 at 12.50pm yesterday.

Fenz shift manager Jules Wevley said a Waiouru and Taihape crew were called to four smaller fires near the railway tracks but they merged into one of about 400 by 20 metres so more crews were called.

A tanker, a helicopter and four rural fire crews helped extinguish the fire.

Wevley said the fire was 80 per cent contained by around 3.30 pm, with crews focusing on extinguishing hotspots.

Helicopter operations stopped by about 5pm and the fire was out by about 6.30pm.

Fenz was in communications with Kiwirail and the railway line was closed as crews fought the fire.

A Kiwirail spokesperson said the North Island main trunk line was closed between Hihitahi and Waiouru from 1pm until 6.45pm.

The line has been reopened today.

Wevley also said fire crews will return to the site today to make sure the fire is fully extinguished.