Eighteen firefighting crews and four helicopters are battling a large grass fire in Glenbrook, near Twizel, in Canterbury.

A Fire and Emergency New Zealand (Fenz) spokeswoman said the blaze was first reported to them at 3.42pm. The first crews arrived from Twizel 10 minutes later.

Fenz estimated the fire covered an area of about 5ha , being 1km by 50m.

Glenbrook is near Twizel and is partly in the Waitaki District of Canterbury.

The MetService forecasted a 28C for Twizel today with mainly fine conditions and northwesterly gusts up to 35km/h. As of 5pm the temperature had dipped to 21C.