However, just after 8am, they got a call to say the same house was again on fire. The house was extensively damaged and will need to be demolished.
Police are investigating the Chef's Kitchen and Seafront Rd fires.
"At this stage both fires are considered suspicious given that both buildings were unoccupied at the time," Detective Senior Sergeant Neil Forlong said.
"There is no indication that these fires were lit by the same person, however."
Police are still investigating a fire that gutted a vacant two-storey property at 255-257 Victoria Ave on October 29. The building was demolished for safety reasons the day after the fire.
A fire that destroyed an empty house in Kaikokopu Rd in the early hours of November 13 was also still an open investigation, police said.
"Police have spoken to a number of people, completed a scene examination and made other inquiries but no one has been arrested at this stage."
Forlong said police encouraged people to be vigilant and call in suspicious behaviour, particularly around unoccupied buildings that could be subject to intentional damage.
Witnesses can report any suspicious activity by ringing Whanganui Police on (06) 349 0600 or call Crimestoppers anonymously on 0800 555 111.