The fire badly damaged the house where no-one was living at the time.
Detective Sergeant Brian McKinney said investigators would like to hear from anyone who saw either a red vehicle, or a dark-coloured twin-cab ute that was towing a trailer.
"The red vehicle was seen leaving Elswick St and heading at speed towards Riversdale."
Waikaia woman Krystal Turnbull discovered the fire as she was driving home about midnight.
As she crossed the bridge on the outskirts of the town she could see a car with its headlights on full coming towards her, she said.
The driver of the vehicle dipped the lights just before it went past her and shortly afterwards she saw the fire.
"You could just see the glow in the background and a wee bit of flames shooting up from the back," Ms Turnbull said.
It looked as if the fire had not long been started but a minute later she could see how much it had spread, she said.
She called 111 to report the fire.
The fire was attended by the Waikaia, Balfour and Riversdale Volunteer Fire Brigades.
Waikaia Station Officer Ray Dickson said when the fire siren went off at 12.04am on Wednesday he looked out his window and saw the house.
"It was well alight then."
The brigade members managed to save a Land Rover in a lean-to beside the garage but there was not much left of the house and garage.
"The vehicles in the garage were lost."
The fact investigators deemed the fire to be suspicious was "doubly sad".
"You hate to think somebody has a reason to burn somebody’s house down."
