Fire north of Kaikoura has sparked the closure of State Highway 1 and the main trunk rail line

Three helicopters will be heading to the blaze shortly.

Fire crews have been monitoring the scrub fire on a farm north of Kekerengu since they were called, about 2am.

Fire and emergency shift manager Simon Lyford says the fire is heading to the coast, and safety concerns have forced the closure of the main highway and railway.

He says heavy scrub and steep terrain are posing the main difficulties.