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Murray Milne-Maresca said it was "too many" and urged members of the community to come forward if they noticed anything out of the ordinary.
Since April 1, there have been 11 structure fires causing damage in Invercargill, 10 structure fires without damage and seven vehicle fires across the Southland region — all of which were treated as suspicious in the first instance.
"We’re worried about them, something’s going on and if someone’s out there lighting fires, well, we want them caught.
"It’s important people keep aware of what’s going on around them."
There was a similar "rush of suspicious fires" in Southland in 1990, Mr Milne-Maresca said.
"It seems to have crept back in but we’re working with police and we’re trying to eliminate [them]."
On Tuesday, Mr Milne-Maresca was sent to a suspicious Hiab truck fire which occurred in a locked compound on Kennington Roslyn Bush Rd, about 12km northeast of Invercargill.
A fire truck and tanker were sent from Invercargill station about 2.15am the same morning and arrived to find the locked vehicle "well-involved" in flames.
Police were in attendance and the fire had been extinguished in about 10 minutes.
"I’ve finished my investigation but the cause is undetermined.
"There was a huge amount of flame and damage done to the truck ... I couldn’t come up with a cause because the damage was so severe."
However, the truck fire was deemed "of a suspicious nature" and would be further investigated by police.
"The truck had not been used for a while, it was locked ... I could not find any volatile, organic compounds ."
While many of the fires were categorised as suspicious, several had been found to be an accident later in the investigations.
"There’s about three or four at the moment where people have been apprehended and they’re going through the court system as we speak."
Police investigations had discovered some of the fires in certain areas around town had been linked, he said.
As a general safety rule, it was a long-standing recommendation that people who owned trucks did not store dangerous materials, such as CRC cans, in the cab of their vehicle but instead, in a locker.
Southlanders were encouraged to implement safety measures to prevent arson attacks by keeping property and vehicles locked, using security lights and keeping flammable liquids, firewood and timber away from areas intruders could access.
He also stressed the benefit of people taking videos or photos to help with investigations.
- Police 105 or Crimestoppers 0800555-111.