Fire and Emergency earlier told residents to keep their doors and windows closed.
A welfare centre for evacuated residents has been set up.
Emergency services responded just before 11pm on Friday, police say.
All four fires have been extinguished.
Fire and Emergency and WorkSafe are both warning of the risks of the heater.
The Government understands Fire and Emergency New Zealand is working with the brigade, Internal Affairs Minister Brooke van Velden says.
Three helicopters were called in at first light to help control the blaze.
Emergency services were called to a property on Fitzroy St in Normanby on Friday morning.
Emergency services rushed to a building on Church St about 1pm.
The road was closed for several hours but has since re-opened.
FENZ said the blaze was "well involved" when crews arrived on the scene.
Fire and Emergency crew are working to rescue someone from inside one of the trucks.
The 650-hectare blaze broke out on the hills last week and crews have been working to get it under control ever since.
The fire, near Waipara, jumped its containment line on Monday afternoon.
Those responsible are yet to be caught, but Police are investigating.
Meanwhile, drones grounded fire crews' helicopter operations again on Saturday and Sunday.
Watch it here.
Firefighters are making the most of a reprieve in the weather on Sunday before strong winds return on Monday.
Emergency services are working to put out the large fire.
Fire and Emergency responded to a well-involved fire just after 3pm Friday.