Auckland Rural Fire spokesman Bryan Cartelle firefighters were on their way from Auckland to replaced those who remained at the scene overnight.
There would be about 40 personnel fighting the blaze today, he said.
"We have got a helicopter doing a flyover currently to quantify the size of the fire and that will determine our suppression strategy today.
"It's still an active fire so it's definitely not contained, and that's our goal today."
The fire was larger than it was last night, he said.
In addition to the 20 homes and factories that were evacuated yesterday, several more homes had been evacuated from the community of Okupu on the island's western coast about midnight.
"They had excessive smoke in that area yesterday and that's why people were concerned," Mr Cartelle said.
The fire flared up about 1.20pm yesterday at the northeastern end of the island, near Claris Airport.
Late last evening the medical centre in Claris and four homes attached to it were evacuated as a precaution.
No one was in the medical centre at the time and St John had received no reports of fire-related injuries from the island.
From above, a trail of damage several kilometres long could be seen cutting through bush on the eastern side of the island.
Many of those evacuated gathered near Claris Airport to watch the fire's progress.
Meanwhile, Northland fire crews are this morning continuing to battle a large scrub fire in Northland.
Fire crews were called to Kelly's Bay, a small beach settlement on Pouto Peninsula, 50km south east of Dargaville, about 9.30am yesterday, a Fire Service spokeswoman said.
The fire covered 35 to 40 hectares at 5.30pm. but had spread across 180ha three hours later.
Rural firefighters backed by four helicopters were continuing to battle the blaze.