Dust - not smoke - appears to have triggered Timaru's first high air pollution night in 2016.
The city recorded its first high air pollution night on March 10, with a PM10 concentration of 55 recorded at Anzac Square.
Environment Canterbury (ECan) measures dirty air in PM10, particulate matter less than 10 micrometres in diameter. Readings higher than 50 are considered to be high pollution levels.
ECan principal strategy advisor Cat White said the concentrations recorded in Timaru were mostly made up of coarse particles and were "very unlikely" to be from burnoffs or home heating.
"We can't say for certain what the source was, but it's most likely to be dust picked up by strong winds," White said.
"This happens outside winter when there is a strong northwest wind. Most high pollution nights are in winter from home heating."
Timaru's first high air pollution night comes six months before the target for high pollution nights is reduced.
White said the target for three high air pollution nights a year would begin after winter 2016.
"Under the Government's National Environmental Standards targets, Timaru's target for three high pollution nights a year begins after this winter – from September this year."
While stubble-burning and outdoor burning of green waste in rural areas had been allowed under the existing Canterbury Air Plan, stricter rules around these activities were proposed under the Proposed Air Plan.
The Proposed Air Plan which becomes operative later this year will introduce new conditions to meet when burning green waste in rural areas.
It also proposes the requirement of a resource consent for stubble-burning inside buffer zones around the towns of Timaru and Ashburton.
Other rules will include requiring farmers to have a Smoke Management Plan for stubble-burning taking place outside of buffer zones, and for any other large scale burn which lasts for more than three days.
ECan is also working closely with woodburner owners in Timaru to show people how to burn better and encourage them to get better burners which are clean-burning.
"We have made some progress in the last year, reducing the number of high-pollution nights from 41 in 2014 to 26 last year, and we know there is a lot more to do to reduce the number down further.
"As part of our summer air programme, we have, since the beginning of summer, been visiting users/owners of wood burners and open fires and talking about how they can achieve a brighter burning fire."
All open fires are prohibited in Timaru.
Compliant woodburners can still be used until they are 15 years old.
If a compliant burner reaches 15 years of age before 2019, it can be replaced with another compliant woodburner, but after 2019 it will have to be replaced with an ultra low emission woodburner.