Two suspicious fires in derelict Christchurch houses are not believed to be linked, but are "almost certainly" arson, the Fire Service says.

Firefighters spent hours battling the blazes, which happened only six hours apart, at properties in the central city last night and Richmond early today.

Emergency services were called to the first fire in Hereford St, next to the Christchurch City Mission, about 9.30pm on Monday.

Canadian tourist Danielle, who did not want her last name published, said she was eating dinner at a nearby motel when she saw smoke and flames outside.

"The whole house was engulfed in fire."

A nearby set of town-houses looked to be threatened by the fire and residents were leaving the buildings when firefighters arrived, she said.

Christchurch City senior station officer Paul Burns said the house was abandoned, but was known to be used regularly by squatters.

Firefighters had been unable to search the house until the fire had been put out.

Fire safety officer Mark Thomas said there had been a "reasonable" amount of combustible material inside the house, including mattresses.

A second suspicious fire in an earthquake-damaged house was reported at 3.30am yesterday.

Firefighters spent four hours battling the fire at a red-stickered Draper St property, with earthquake damage to the house making it too dangerous for them to go inside.

Thomas said both fires were "almost certainly deliberately lit", but there was nothing to indicate the two fires were linked "apart from the fact they happened on the same night".

Both houses would have to be demolished, he said.

"The [Draper St] house is extremely badly damaged previously by earthquake damage ... and one of the rear walls collapsed, so it’s going to have to go soon."

Police were investigating the fires.

- © Fairfax NZ News