Ships wishing to re-fuel or ‘bunker' at Port of Tauranga are being forced to use the cement/tanker berth at the southern end of the Mount Maunganui wharf while the cause of last Monday's oil spill is investigated.

A small hole in a spur line off the main under-wharf bunker line leaked an estimated 1500 litres of heavy fuel oil into the water on April 27.

Mobil personnel are assisting with the clean-up. Photo: Supplied.

Carried by wind and the tide, the slick polluted the Tauranga Bridge Marina and the upper harbour at Maungatapu.

Mobil is importing two pipeline experts, due to arrive from the US today, to assist with the investigation into the incident.

They will work with Mobil Oil New Zealand technical experts on site, and will fully cooperate with Bay of Plenty Regional Council staff as they undertake their investigation consistent with their role under the Resource Management Act.

The priority is to understand how the failure occurred. Further metallurgical testing of the pipe will be required to understand the specific cause, and is expected to take some weeks.

The pipeline system to the wharf remains shut down while inspections and testing are carried out on all sections of the pipeline branches along the wharf to ensure their ongoing integrity.

Mobil personnel are also assisting with the clean-up of remaining traces of fuel oil around the wharf where the leak occurred, with a team of people working over the weekend alongside council personnel to remove oily debris and small pockets of oil.

Absorbent booms remain in place, however this area is now substantially cleaned up.

Mobil is working with the Marina and boat owners to provide help and advice as the cleaning of boats impacted by oil gets underway.

They are also assisting where they can with the ongoing clean-up of the marina area, where a sheen is still visible in the water in some areas.