There have been 47 oil spills reported in Tauranga Harbour in the past two years, but only one case is being prosecuted.
Community leaders have called the new figures concerning and unacceptable, and a local iwi representative says "recklessness" is to blame.
The oil spill data was revealed to the Bay of Plenty Times in a Local Government Official Information and Meetings Act response by the Bay of Plenty Regional Council.
It showed sources were not found for 29 of the reported oil spills, including the one at Matapihi last month.
Five of the 47 reported spills were estimated to be greater than 50l in volume, seven were between 20l-50l, and 21 were between 1l-10l.
The remaining 14 were classified as "nothing detected or quantifiable".
Carlton Bidois, Ngāti Ranginui representative and member of the council's oil pollution response team, said the figures showed how "reckless" some commercial and recreational boat owners and companies were.
"The interesting fact or figure would be how many of those people who were responsible for those spills, if any, owned up to it, or only found out because the regional council did an investigation and tracked it back to them."
Bidois said it did not matter whether it was 1l or 300l spilled.
"There should be no accidental spills. All mechanisms are in place, the law is in place. It's recklessness that's caused it. So that's the worry for me."
He said the number of reported spills was climbing and that was a serious concern for iwi and hapu.
It was one of a few ongoing threats, including expanding industry, having a cumulative impact on the harbour and the region's waterways, Bidois said.
"We've lost so many significant shellfish species in the past five to seven years it's not funny. It's absolutely the truth that when we have that sort of effect, it does affect our culture."
Bay of Plenty harbourmaster Peter Buell said the council responded to every reported spill and its priorities were to stop further oil entering the harbour, capture what was there, clean up, gather evidence and investigate.
"We investigate every report of an oil spill on water but, unfortunately, it's often not possible to trace back to a source."
He said determining the volume of a spill could also be difficult due to the varying nature of different oils, some of which break down much faster than others. Conditions also played a big part in the speed of oil dispersing.
A source was determined in just 18 of the 47 reported spills and action was taken in 12 of those cases.
Five were found to be accidental and warnings were issued, two received infringement notices, four resulted in costs being recovered and one is going to prosecution.
Buell said while it seemed like the frequency of spills may be increasing, the public was doing a better job reporting.
He said the council was also becoming more proficient in letting the public know when spills occurred, no matter how minor.
"We learned some valuable lessons from the Mobil spill where we didn't do as good a job on that aspect as we would have wanted to."
Greg Milne, Matapihi Residents and Ratepayers Association chairman, said locals were angry and upset by last month's spill, which was estimated by the council to be 20l-50l in volume.
"It's a well-used bay and anything that pollutes it is bad. The feeling in Matapihi if there's oil or any other pollutant on our beach ... there's anger and frustration, particularly if you can't find the source of it."
Milne was surprised by the number of spills in Tauranga Harbour in the past two years and said it was "very disappointing" and "unacceptable".
But Dr Ian McLean, Toi Ohomai Institute of Technology associate professor in resource management, was not surprised by the oil spill figures.
"You know that these things are happening and you know that most of them are small and are unreported."
He said most spills were likely unintended and carried minimal impact and so a low number of prosecutions was both expected and appropriate.
McLean thought the number of spills in which a source was found was reasonably high and he was impressed by that.
However, he said more work needed to be done when it came to monitoring and considering the cumulative effect of small spills on ecosystems.
Tauranga Bridge Marina's Anna Barnes said the Mobil oil spill in 2015 had a detrimental effect on the marina.
"I am reassessing our Clean Marina status at the moment and one area we struggle with is offering an option for boaties to dispose of their oily/dirty bilge water and expired petrol."
Barnes said, with more than 2000 boats in Tauranga Harbour, there should be a readily accessible facility to take the waste.
"This would encourage boaties to clean their bilges regularly which may otherwise automatically discharge into the marine environment after rain."
Regional councillor Matemoana McDonald, who represents the Mauao Māori Constituency, said the new oil spill numbers were high and were naturally very concerning.
However, she supported the explanations provided by the council "because only by our working together can we start to improve on these current stats".