At least seven, mainly small fires, in Hastings and Napier in less than four weeks are now being considered as possible arson.
Detective Ryan Honeyfield says the five smaller fires in Hastings could possibly be linked, happening around the suburb of Frimley.
Four were set in bushes, in one case an elderly man knowing nothing of the fire outside until he awoke one morning to discover the trees at the front of his property had been burned.
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The first was in Lyndhurst Rd on March 16, two were at the same Nottingley Rd property on the night of Christmas Day and early morning of December 30.