Oil has been spotted near the Rena wreck at Astrolabe Reef, where the stricken cargo ship grounded more than three years ago.
Eyewitnesses say the spills covered large areas but officials say the sheen from the spills dissipates almost immediately, suggesting small quantities of oil.
A Tauranga pilot said a large spill could be seen from the air on Friday. He estimated the oil stretched over an area of about 12km.
"It's not all joined as one. It's just big blotches everywhere," he said.
"I've never really seen it bad but it's definitely the worst I've seen myself."
The man, who spoke on the condition of anonymity, said the oil was located about 3km west of Motiti Island and 3km from the Bay coastline.
The observation comes just days after a man who is part of the recovery team told the Bay of Plenty Times oil had been leaking for the past few weeks.
The man, who also did not want to be named, said oil had leaked onto the water's surface on Tuesday.
He was compelled to speak out because "people deserved to know", he said.
"I can't sit back and let nothing be said about it."
The man said he believed hydraulic oil was leaking from machinery still onboard the submerged wreck.
"Every time the water gets up it seems to set it off."
Salvors were not at the wreck on Thursday due to rough sea conditions but some oil was still visible, despite choppy conditions, when the Bay of Plenty Times visited the reef. The man said he was concerned at the oil and 261 containers that still remain on board.
"I've seen it on a daily basis ... there is still a heck of a lot of oil in there. I'm talking about tonnes, not litres."
Another man who works as part of the recovery project said oil leaked "all the time" and there was often large pools of the substance floating around the area.
"There's been oil leaking for a long time. You would see it from some distance away. It comes at different times but when it gets choppy it tends to break it up."
In a written response, Hugo Shanahan, spokesman for Rena's owner and insurers, said Resolve salvors had noted that trapped pockets of residual oil would be released as part of the recovery operation.
"Sightings so far have all risen to the surface and dissipate[d] before evaporating very quickly - confirming that these deposits are in very small quantities."
Maritime New Zealand spokesman Steve Rendle said: "Maritime NZ has been advised that a very small sheen was seen after debris was brought to the surface by salvors on Monday but this dissipated and evaporated almost immediately."
Mr Rendle confirmed a total of less than two tonnes of oil was estimated to remain on board the submerged wreck.
Bay of Plenty Regional Council is responsible for managing any oil discharges from Rena.
Deputy chief executive Eddie Grogan confirmed an oil sheen had been reported to them on Tuesday.
"The sheen's location was GPS tagged. However, the amount of sheen was so small that it dissipated before they could walk from one side of the barge to the other to take a picture of it," Mr Grogan said.
"We think this sheen may have been associated with debris removal from an area where oil was not previously stored. It may have been clinging to unrecovered debris exposed during the recovery operation."
Mr Grogan said salvors Resolve Fire and Salvage was careful to report every amount of oil they sighted and had the capability to respond immediately to a spill.
Bay of Plenty Regional Council was also ready to respond to any spill, Mr Grogan said.
If anybody saw oil on the shoreline, it could be reported to the regional council Pollution Hotline 0800 884 883.