They have not linked the fire, which started about 8.20pm on Saturday, with two gang-related arsons on Thursday.
Fire on Thursday destroyed the Balmoral Dr headquarters of the Road Knights gang.
But police are treating the Saturday night fire as suspicious and, although yesterday they would not say what caused it, they said an accelerant may have been used.
It appeared the address was linked to the Mongrel Mob only by its physical proximity. Police were yesterday attempting to locate the owner of the property.
Senior Sergeant Olaf Jensen said the inquiry was in its early stages. Senior Constable Dave Loader said a scene guard had been in place at the Mongrel Mob headquarters earlier in the week and police patrols had not noticed anything suspicious in the area on Saturday night.
Police were not aware of any party taking place at or near the house on Saturday night, he said.
Senior Sergeant Mark Lucas said Invercargill police were not aware of anything that would have sparked tension between the two gangs.
Snr Sgt Jensen said police wanted to assure residents the force had additional personnel present to prevent an escalation in gang tension.
Police patrols had increased since the first fire, and police were highly visible in the cit
