A scientist who worked on the study said the high levels in Customs St, which is within 200m of the waterfront and its sea breezes, suggest Auckland is becoming complacent about advantages provided by its climate and geography.
"Generally, Auckland air-quality levels measure well against the rest of the world - our background levels on the city edge are among the best in the world," said Ian Longley, who co-headed the study. "So we have a natural advantage which we are squandering to some extent."
The air was monitored in a study by the National Institute of Water and Atmospheric Research with scientists from the University of Auckland and AUT University.
The project measured an average of almost 45 micrograms of nitrogen dioxide for every cubic metre of air across nine sites along the street over six weeks, compared with a maximum annual mean level of 40mcg in WHO guidelines incorporated into Auckland's long-term planning rulebook.
An average of 59mcg was recorded at one site, alarming the Heart of the City business association, which co-sponsored the $200,000 study with Auckland Council's CBD advisory board through a targeted rate on downtown property owners.
Nitrogen dioxide is a pollutant from diesel engines, making buses converging on Britomart prime culprits - and prompting a call from the business association for a rethink of transport routes.
"People will be getting out at Britomart from the new electric trains into the polluted environment of Customs St," said association spokesman Greg McKeown, a former Auckland City transport chairman who is lobbying the Super City council for matching funds for a follow-up study.
"You can't shift the people but you can spread the bus routes around."
Council infrastructure chairman Mike Lee said the results showed a need to reconsider introducing electric light rail to Queen St and its surrounds rather than double-decker diesel buses favoured by planners.
On the other hand, the study found concentrations of carbon monoxide from petrol cars were "far below any relevant standards or guidelines".
Dr Longley, Niwa's air quality science leader, said that was unsurprising, given the prevalence of carbon-filtering catalytic converters in cars.
He emphasised the nitrogen dioxide findings were indicative only, involving "relatively low-cost, crude technology". More sophisticated equipment would be needed to be definitive.
"What we would say is that this methodology gives an early warning."
