A passing motorist noticed the fire on Paekakariki Hill Rd about 3am.
The owners of the house are overseas.
The 570sq m home had been renovated over the past year and had cost a fortune, said the owner’s brother, Nigel Welch.
Detective Sargeant Steve Dunn confirmed the fire was being treated as suspicious.
He asked anyone in the area who had been cold called recently to contact Porirua police.
Porirua senior station officer Owen Woodman said no-one was inside the house when the fire erupted.
“It was initially believed there were, but fortunately it was empty.”
Damage to the house was extensive and police and fire investigation officers were now looking into the cause of the fire, Mr Woodman said.