About 1500 litres of heavy fuel oil was leaked into Tauranga Harbour on Monday, Mobil has revealed.
The news comes after oiled debris washed up at Pilot Bay yesterday.
Mobil told the Bay of Plenty Times Weekend on Thursday the volume of the spill would be hard to determine because the leak in the pipe was located before the fuel meter and a spokesperson said it would be a week or two before engineers could estimate how much fuel was spilled.
Mobil New Zealand country manager Andrew McNaught yesterday sent a statement to media that said: "While it is difficult to make an accurate assessment of the volume lost given the circumstances, Mobil estimates the volume of fuel leaked into the harbour to be approximately 1500 litres."
Mr McNaught was in Tauranga yesterday meeting with Bay of Plenty Regional Council staff and said he would like to thank the teams at the regional council, the Port of Tauranga and all others involved over the past few days, for their "outstanding efforts" with the response and clean-up.
He said the investigation was continuing and the priority was to understand how the failure occurred. "Further metallurgical testing of the pipe will be required to understand the specific cause, and this will take some weeks."
Mr McNaught said Mobil staff were assisting with the clean-up of remaining fuel oil and were working at the Tauranga Bridge Marina providing help and advice for the cleaning of boats impacted by oil.
Iwi representatives spoken to by the Bay of Plenty Times Weekend were upset they had to hear of the oil leak through media.
Buddy Mikaere, a prominent campaigner for removing the Rena on behalf of Motiti Island iwi, said after the Rena wrecked, the regional council and Maritime New Zealand put together a response strategy about involving iwi "right from the get go" after an oil spill.
"I'm amazed that at its very first test, it fell over. If it wasn't for the media, we possibly still wouldn't know what happened.
"Why put all that effort into sorting out the response plan when it was obviously just a tick the box response? I'm just a bit disappointed to be honest."
Mr Mikaere said the regional council had done a good job "picking up the ball after the fact and running with it" and had involved iwi in the clean-up.
Charlie Tawhiao, Ngai Te Rangi chairman, said he had first heard about the spill in the media, which "wasn't the best way of us maintaining the relationship with various bodies".
Mr Tawhiao said the wounds were still raw after the Rena both with Maori and other locals.
"It's hard for [people] to have any confidence in our systems when this keeps happening. For the people who went through Rena, it's their worst nightmare."
There was 1700 tonnes of heavy fuel oil on board the Rena.
Bay of Plenty Regional Council strategic communications manager Sue-Ellen Craig said there had been a delay in informing iwi due to a system error.
"The initial notification was intended to go to iwi and other key stakeholders, unfortunately this was sent remotely outside of our usual system due to it being a public holiday. This process has now been rectified to prevent it occurring again."
Bay of Plenty Regional Council on-scene commander Jim Tetlow said the council's staff were currently focused on the clean-up.
He said at the end of Thursday, 12 tonnes of oily waste had been removed from the foreshore at Maungatapu and Motuopuhi (Rat Island).
"We have more clean-up work planned for the weekend, focusing on the port now that wind conditions are more favourable for on-water recovery ... We also had reports of some oiled debris at Pilot Bay [yesterday] afternoon and have a crew cleaning that up now.
"We'll re-assess what further work is needed and how much longer it will take on Monday."