The Department of Conservation (DOC) is stepping up its vigilance around a kiwi sanctuary on the West Coast after a fifth deliberately lit fire was discovered overnight.

The fires were lit near the Okarito Kiwi Sanctuary, about 25km north of Franz Josef. The sanctuary is home to the rarest kiwi species, the rowi.

The first four fires were spotted from a helicopter about 3am yesterday, with the largest spreading over an area of about 500sqm.

A helicopter with a monsoon bucket, firefighters and DOC staff tackled the blazes.

Another suspicious fire was discovered overnight by the side of State Highway 6 and staff had stepped up their vigilance as a result, DOC programme manager Julian Tovey told Radio New Zealand.

"Obviously with another one last night we're on the game now, just trying to watch out really."

It was still unclear why the fires had been lit.

"There's always people with grudges against DOC for different reasons," Mr Tovey said.

"It could just be hunters trying to get some areas of regrowth so deer come in and then they can have a hunt later on. So there's different theories on why, but we've no strong leads at the moment."

One of the fires yesterday had encroached within the boundaries of the northern end of the sanctuary and could have put the rowi at risk.

Nesting had finished for the birds and hopefully they would have been able to escape the flames, Mr Tovey said.

Staff would this week assess whether the rowi population or environment had been affected.

The total rowi population is about 350.

The area had seen other arsons in the past, normally about this time of the year when manuka trees were dry and could catch fire easily.