Christchurch residents face a fine of up to $300 if they use open fires or old woodburners between now and the end of September this year.
The Canterbury Regional Council ban has the support of health advocates, but there is concern about keeping homes warm during winter.
The Canterbury District Health Board says the law will significantly reduce the amount of air pollution, which causes up to 70 deaths in the region every year.
But Community Energy Action's chief executive, Bede Martin, says if people on low incomes cannot afford the increased electricity costs, they may go cold.
Mr Martin says it will particularly affect tenants whose landlords refuse to install a suitable alternative form of heating.
He hopes landlords will take advantage of subsidies, which help with the cost of switching to a cleaner heating device.