Wellington's air quality has been declining despite the city's famously windy conditions, according to a report released by Greater Wellington Regional Council's Environment and Climate Committee.

The central city has recorded a 15% increase in black carbon levels on Manners Street between 2023 and 2025, with diesel buses identified as the primary cause. Earlier gains along the Golden Mile retail and transport corridor are now being undone.

GWRC councillor Quentin Duthie said growing passenger numbers are being met with additional diesel buses because council funding for electric fleet expansion remains constrained. Since 2020, when a quarter of diesel buses were swapped for electric vehicles, black carbon and nitrogen dioxide had fallen by almost half.

In Wairarapa, wood burners continue to drive poor air quality. Masterton and Carterton each logged more than 50 high-pollution days annually, against World Health Organisation guidance of no more than three such days per year. The report estimates Masterton will need 20 years to meet WHO particulate matter thresholds.

University of Otago associate professor Caroline Shaw, a public health specialist, said air pollution affects every organ and can trigger asthma, heart disease, cancers, diabetes, dementia and pregnancy complications. Human-made air pollution causes 3000 premature deaths in New Zealand each year.

"The only safe level from air pollution is actually no air pollution," Shaw said.

GWRC chair Daran Ponter said the council is obliged to address pollution under the Resource Management Act and in its role as public transport provider. Potential responses include expanding the electric bus fleet and rerouting some services along the harbour quays, away from the Golden Mile.

Report co-author Tamsin Mitchell, an air quality scientist, encouraged residents to adopt low-emission heating and transport. "You've got to enable people to be able to change and you've got to support them in that change and you've got to make sure there's no perverse outcomes - like they're living in a cold home," she said.