"There is no minimum safe exposure limit for fine particulates, so any exceedance of 50 is considered quite serious," said Karen Parcell, acting manager of natural resources policy at the regional council.
The data shows the first breach of air standards since July 2014, fitting into an established trend of hazy pollution during the colder months.
There does appear to have been some improvement in Rotorua's air quality since last year, although it is hard to view the pollution drop as part of a longer-term trend.
Average daily PM10 levels between May and July this year were 15.86 per cent less than the same period in 2014 - but barely less than those recorded in 2013.
The regional council claims Rotorua has the worst winter air quality in the North Island, with home heating accounting for 60 per cent of the city's pollution.
It cited this unflattering reputation when introducing a ban on the use of open fires in the city's urban airshed that came into effect on May 1, despite requests from the Rotorua Lakes Council to delay the measure because of a claimed lack of public awareness.
At the time, news of the ban was met with frustration by some Rotorua Daily Post readers.
"My fire heats my water, [so] if and when we have a power cut, I will have warmth, light and [be] able to cook some food so I am not hungry or cold. Who's idea was this?" read one Facebook comment. To ease such concerns, the regional council promised it would take a lenient, "educational" approach during the first year of the ban.
Ms Parcell said no active recording of infractions or actions has been carried out since May 1, and she was not surprised the ban had failed to prevent the annual breaches of pollution levels.
Data from other areas of the country suggested it would take two years for such a ban to have a significant impact.
"As more and more houses become aware and start changing their behaviour, we should see a drop in the level of fine particulates."
Regional councillor for Rotorua general Neil Oppatt said initiatives such as incentives for residents to replace open fires with cleaner heating systems were starting to prove effective in reducing pollution.
But he conceded there was still some way to go.
"The numbers are down and we are making big improvements," Mr Oppatt said.
"But we need to convert a lot more [open fires]."
Mr Oppatt said the regional council was restricted in its ability to tackle some of the other causes of poor air quality pollution - including vehicle emissions, which caused 12 per cent of Rotorua's pollution.
"That's for central government to decide," said Mr Oppatt.