Fire crews are battling a large structure fire in McNulty Rd, Cromwell tonight.

Eight trucks were called to the fire in a commercial building about 4pm and were still battling the blaze at 8.30pm.

Crews have attended from Cromwell, Dunstan and Alexandra volunteer fire brigades and a digger and loader had also been brought it to help.

Fire and Emergency New Zealand southern fire communications shift manager Simon Lyford said crews would remain on site well into the night because of the complex issues they were dealing with.

Although they had a lot of work ahead of them, they were making "good progress", he said.

"The issue with the fire is it's inside a large commercial building."

He said damage to the roof meant crews were fighting the fire from outside the building.

Residents are being asked to keep their windows and doors shut to minimise smoke drifting into their homes.

Anyone experiencing respiratory issues should call Healthline on 0800 611 116.