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Emergency services were called to a property on Newcastle St about 5.45am on Sunday after a neighbour saw flames.
The fire was extinguished by around 8am, but the house was "extensively damaged as well as the car parked beside it", a Fire and Emergency New Zealand spokesman said on Sunday.
No-one was injured as the house was unoccupied at the time.
Police today said initial inquires indicated the fire was deliberately lit.
Minutes before the blaze took hold, the neighbour who saw the flames heard glass breaking and a vehicle leaving at speed.
Police are asking any local businesses with CCTV footage around the time of the fire to get in touch.
Anyone else with information is urged to call Alexandra Police on (03) 440 2500 or Crimestoppers anonymously on 0800 555 111.