The blaze at Sulphur Flats on Monday night was not a sulphur fire and is being treated as suspicious.
Fire safety investigator Stuart Bootten said they had found the point where the fire started and there was nothing obvious that would be a natural cause, therefore they were treating it as suspicious.
He said he could not speculate as to whether the earlier fire that day near St Michael's Rugby and Sports Club was related but if anyone had any information about either fire, it would be more than welcome.
"The fires are destroying the vegetation which is a shame ... it kills the regeneration of the vegetation which is a big disappointment," Mr Bootten said.
He would be passing the investigation on to the police when he had finished his investigation.
"It could be a fire bug, but it could be a one off, that's the million dollar question."
He said sulphur fires started on Monday night in the area as a result of the vegetation fire.
Station officer Tony Kelly said the vegetation takes years to grow back.
"It's destroying something nice here, it's not often that you can run or bike through a city and of course the outside of the gum trees burn extremely well and really fast."
Mr Kelly said in areas near Sulphur Flats they always treated fires as suspicious because of the number of fires they had had and because they had caught people before.
"With the two sets of fires from (Monday), that is definitely suspicious. I wouldn't like to say there is a fire bug around. If there is, then they are destroying their own city. It's a shame that they want to burn this, that's what the disappointing thing is. It ties up resources as well," Mr Kelly said.
He said because we were such a geothermally active area, on hot days or when any nearby fire heated up it could start sulphur fires.
"It's the build-up of the heat in the ground, they are really dangerous. They ignite all the ground on fire. The ground gets really hot and opens up a gap of air to get into the sulphur and the fires start.
"In sulphur fires, the flames are all blue, you can't see a sulphur fire during the day, you can only see them at night."
Rotorua Lakes Council rural fire zone manager Richard Horn said they had a crew of about five there from 8am yesterday dealing with hot spots.
"I'm comfortable that the fire is out. We will try to do our best to monitor the area."
He said if the public saw anything out of the ordinary, they needed to call 111.
Detective Senior Sergeant John Wilson said a 13-year-old was dealt with at the end of last year by the Youth Aid Section over the arson in November.
He said at this time of year, the issue was that everything was so dry.
"If any kind of fire started, whether it be lit on purpose or not, it is putting people at risk who are using that area legitimately and it's also putting at risk the people who are having to put the fire out.
"It's not just a bit of waste ground. It has the potential to spread to houses and other buildings so basically it is a real concern."
Mr Wilson said fires were not always lit by bored youngsters.
"Over the years, I have found there are lots of motivations for people to light fires so we make no assumptions."
He said whenever someone broke the law, others generally knew about it. "If you do something wrong, someone usually sees something. People do notice. So our message is see something, do something."
Additional reporting by Kelly Makiha
Recent suspicious fires:
* Fire at Sulphur Bay on November 8, 2015
* Fire at Linton Park on December 10, 2015
* Fire near St Michael's Rugby and Sports Club on January 25, 2015
* Fire at Sulphur Flats on January 25, 2015