A St John spokesman said an ambulance was called to the Insoll Ave blaze about 10.20pm.
One patient was taken to Waikato Hospital, where a spokeswoman confirmed they remained in a critical condition in the intensive care unit.
Northern Fire Service spokesman Colin Underdown said two fire trucks rushed to the scene to assist ambulance officers, after receiving a call about the person's burns.
He said the fire was outside the house and believed to be a bonfire, but had been put out by the occupants.
"There was a person removed from [the] scene by ambulance," he said.
"We just got a call to someone with burns so we just sent two trucks thinking it [might be] a house fire but when we got there we got stood down. I think it was a bonfire."