A ship re-fuelling at the Port of Tauranga spilled an unknown quantity of heavy fuel oil into the harbour last Wednesday.

Bay of Plenty Regional Council pollution prevention manager Nick Zaman today told SunLive authorities are unsure of the amount that spilled into the harbour from the Singapore flagged cargo ship Liloa.

Singapore flagged cargo ship Liloa was refuelling at the Port of Tauranga when oil ended up in the harbour during refuelling.

The island trader was re-fuelling at No.1 berth at the north end of the Mount Maunganui wharf on July 3 when fuel spilled onto the ship's deck and into the harbour.

Regional council were alerted to the spill of heavy fuel oil around 4.25pm and were on scene in less than 10 minutes, says Nick.

Regional Council Maritime and Pollution Prevention staff worked late into the night and the next day cleaning up.

An inspection of the beaches at first light found oil pollution along the south of Pilot Bay by the toilet blocks to Salisbury Wharf - the area around the berth at the Port.

Nick says oil was located in eel grass near the Pilot Bay toilet block, which was bagged and removed. There is no discernible tideline on moored boats in the bay and a shoreline search of the entrance in daylight on Thursday found minimal oil.

Apart from some spots of oil around Pilot Bay, minimal oil was observed, says Nick. There were no observed environmental effects or reports through the Regional Council's Pollution hotline.

'We went round and looked and didn't see anything other than what was reported. The tide was on its way out at the time. It wasn't very windy,” says Nick.

'The larger quantity might have more than likely gone out to sea with the tide.”

A very light sheen was seen on water near the berth the next day, and a patrolling boat witnessed beads of oil in a tidal eddy. Peat was added to the oil to take it out of the water column so it can degrade naturally, says Nick.

The ship was allowed to sail, and further crew interviews took place in Auckland on Friday before it departed for Nuku'alofa.

Nick says the investigation is continuing. A decision on prosecuting has not yet been made.

When asked today why the public weren't told about the spill and the clean-up the following day, the regional council's senior communications advisor Linda Thompson says there will be a press release on that.

'I've just been talking with the CEO about that, so we will have something for you shortly,” says Linda.