The fire is over an area of about 150m by 15m between part of the wastewater treatment plant fence line and Puarenga Stream.
Fire Service assistant area commander Brendon Wood said a specialist fire investigator was at the scene working to determine the cause.
He said the blaze was contained within 20 minutes but because of the size of the fire, there was a lot of dampening down to do.
He said there were also a couple of sulphur fires across the river that were unrelated.
"But it just goes to show the volatile nature of the terrain, that's why we can't determine the cause until we put the fire out to give someone a chance to get in safely."
Only minutes after putting out a fire off Old Te Ngae Rd, firefighters were called to the bigger blaze.
Reporter Kyra Dawson at the scene said the blaze was a lot bigger than the earlier fire and large plumes of smoke could be seen.
"There is a lot of dark smoke. The fire trucks were having trouble getting access. There is just heaps of smoke."
Cheryl Broock, who is sitting on the hill near the fire, said she lived nearby and could smell the fire.
"It is sad if it's another arson case," she said.
Northern Fire Communications shift manager Steve Smith said the scrub fire started at 5.30pm.
There are currently six fire engines there fighting the blaze.
St John Ambulance paramedic Mike France said: "We are on standby because it is a major event, mainly for (the safety) of the fire guys."
Another witness Myrin Abbott said fires always happened in the area.
"It's just part of how dry it is... It's a decent one."
Okareka volunteer firefighter Stuart Pinfold said there was a big team working to put out the blaze.
"It's pretty much extended all of our resources in Rotorua."
