Air quality researchers say woodfires in homes could expose residents to unsafe levels of air pollution.
Scientists led by the National Institute of Water and Atmospheric Research have been measuring the smoke from woodburners in three houses in Canterbury and Auckland.
Researcher Guy Coulson says the team measured concentrations of the tiny particles present in smoke and soot.
Normally taken outside, readings that average above 50 microgrammes per cubic metre are considered unhealthy.
Dr Coulson says the tests returned short 10-minute bursts of three to four milligrammes of PM-10 per cubic metre, which is close to 100 times the outdoor standard.