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A veteran Dunedin chimney sweep has shared his tip for avoiding dodgy wet wood this winter.

Otago Chimney Cleaning Services operator Murray McEwing has been cleaning and servicing fires in the city since 1996.

• Dirty chimney fire hazard warning

For about $40, he bought a wood moisture meter from Mitre 10, and said he had measured firewood bought from suppliers that had a moisture content percentage in the high 20s and above, meaning it was not properly seasoned and full of water.

''It won't combust properly and as the gases are going up to the flue, and the flue's getting cooler and cooler and cooler, by the time it gets to the cap those particles are ... condensing and it'll build up,'' he said.

Ideally, wood would have a moisture content of under 21%, he said.

The consequences of wet wood could be not just a mediocre fire, but chimneys possibly clogging even before winter was over, he said.

''I've seen them build up in 2-3 months, from the time I've gone and cleaned it.

''They've got some wood, it's wet, and their fire's just completely stopped and they're ringing me up. I take videos and photos and take it down to them and say, 'this is what it is'.''

In his view, an annual clean should suffice for most fires, but for some ''stay-at-home mums and older folk'', their chimneys and flues would require more regular cleaning.

Mr McEwing said he once arrived at a job in Shetland St where a chimney fire had sent sparks flying into a macrocarpa hedge, showing dirty chimneys and flues could have far-reaching effects.

''That's a real problem. You may not do any damage to your house ... but the sparks could set the neighbour's hedge on fire.''