There were 18 oil spills reported in Tauranga last year and one in the Whakatāne River, according to new data from the Bay of Plenty Regional Council.
A source was identified in just five of the 19 spills.
Two resulted in warnings being issued to recreational boaties, and three were sourced to commercial transfers of hydrocarbons (such as fuel or oil) in the Tauranga Harbour, the regional council said.
Last year, more than 870 million litres of hydrocarbons were transferred at the Port of Tauranga from ship to shore, where it is stored in tank farms, and shore to ship.
The year before, it was more than 911 million litres.
One of the spills from a commercial transfer last year resulted in an infringement notice being issued and the cost of the spill being recovered.
The regional council said there was no further action taken on the other two reported spill incidents involving commercial transfers "due to the nature of the events, i.e. they were well responded to and contained".
A Port of Tauranga spokeswoman said, in one of those three incidents, "a small amount of lubricant" was spilled when a valve failed during a transfer to a ship at the Mount Maunganui wharves.
The company involved cleaned up the spill on land and in the water using absorbent and booms, with assistance from the harbourmaster, she said.
"The other two incidents involved less than a litre of hydrocarbon on land. No product entered the water and they were quickly cleaned up in accordance with the site spill procedures."
The spokeswoman said the port took spills "very seriously" and had a range of rules and strategies in place, including environmental management and response requirements for all companies operating on the port, specific procedures and plans for users transferring hydrocarbons to and from ships (which have to be approved and involve annual spill tests/exercises), and education and training exercises around spill minimisation and response.
Every reported environmental incident is investigated and the port's tanker berth is a specially designed sealed and bunded area to ensure spills in the area can be contained.
Altogether, the 19 reported spills in Tauranga and Whakatāne between July 1, 2018 and June 30, 2019 had a total volume of about 143 litres.
In terms of the clean-up involved, 15 of the 19 were estimated to be between 1 litre and 20 litres in volume and three were between 20 litres and 50 litres (including the one in Whakatāne). One was contained to a concrete wharf and did not enter the water, the council said. No estimated volume was given for that spill.
There were three reported spills last year listed as land-based, which the council said was usually stormwater run-off. The others were on-water only.
Carlton Bidois, Ngāti Ranginui representative and member of the council's oil pollution response team, said it was always hard to trace an oil spill and the high number of unsourced spills last year suggested people were not admitting when they were at fault.
He said the public and commercial parties were doing a better job of reporting spills, and the regional council's oil response team was doing a better job policing them.
"But it's obvious that the boat owners themselves have still got a lot of work to do. If the individual admits when he spilled it, well then, they're going to find the source," Bidois said.
"No oil spill is acceptable. It's more around being reckless than accidental. People just need to be more vigilant, stay vigilant, own up to what they're doing and put the right mechanisms in place. And use your common sense. Use the rules."
Bay of Plenty harbourmaster Peter Buell told the Bay of Plenty Times that while he was pleased to report both the number of spills and volume of hydrocarbons spilled were both down this year compared to last, "no oil spill is acceptable".
He also said the numbers did not tell the full story, "as no two spills are the same".
"For example, factors such as better promotion of the Pollution Hotline leading to more spills being reported and the fact that there are more boats taking to the water than ever before need to be considered."
Buell said the council responded to reported spills when the spill exceeded the clean-up capability of the person responsible for spilling the oil, or where the person responsible for spilling the oil could not be identified.
In 2017/18, there were 33 reported spills in Tauranga; totalling about 527L. In 2016/17, there were 14 reported oil spills.
A source was determined in just 18 of the 47 reported spills in those two years 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 went to prosecution.
In December last year, the Bay of Plenty Times reported a Bay of Plenty company was fined $40,000 after almost 150 litres of hydraulic oil spilled into the Tauranga Harbour.
• Anyone who sees oil on the water can call the 24/7 Pollution Hotline on 0800 884 883.
2018/19: 18 reported oil spills in Tauranga, plus one in the Whakatāne River.
2017/18: 33 reported oil spills in Tauranga.
2016/17: 14 reported oil spills in Tauranga.
Source: Bay of Plenty Regional Council