Ocean currents have carried oil leaking from a shipwreck on the Solomon Islands away from a nearby world heritage site as authorities continue a clean-up operation that is expected to take months.

Cleaning up after the bulk carrier MV Solomon Trader, which ran aground on a coral reef in early February and released 80 tonnes of oil, is expected to take up to four months.

But the slick has been pushed away from the island’s eastern flank which is a world heritage site. Rennell Island is the the largest raised coral atoll in the world.

Australian authorities, who kicked off the initial on-water clean-up, have now retreated and the ship’s Korean insurer has taken charge of the operation. There will be ongoing surveillance flights carried out to monitor the process.

MV Solomon Trader had been loading bauxite from a mine in the days before rough seas pushed it aground on a coral reef in the early hours of 5 February, in the lead-up to Cyclone Oma.

The clean-up operation of the oil slick could take between three to four months, an expert said, while the ship wreck itself could be removed in the next four to six weeks.

It is unknown what percentage of oil has been cleaned up so far and the amount removed from the water each day varies from dozens of tonnes to single digits.

A barge pump has been steadily removing the 600 tonnes of oil still onboard the vessel. About 300 to 400 tonnes has been removed so far.

The Solomon Islands Maritime Safety Administration said its investigation into the incident was ongoing and officials were trying to get access to the ship’s voyage data recorder. “This should shed some light on the circumstances of the incident,” acting director Jonah Mitau told the Guardian.

The administration’s preliminary investigations indicated there may not have been enough crew members on night watch when the ship ran aground during a cyclone.

The Hong Kong owner King Trader apologised over the disaster but insisted reports of the Solomon Trader “crew being absent from the vessel or intoxicated at the time of the grounding are false”.

The damage bill for the clean-up is still expected to be in the order of $50m.

Residents on Rennell Island have reported some health issues as a result of the oil spill as well as disruption to their fishing and swimming activities in the bay.

Rennell is the largest raised coral atoll in the world, and contains diverse forests and coral. Unesco has listed it as a World Heritage site in danger since 2013.

Solomon Islanders go to the polls on 3 April. Shipping law reform is expected to be on the agenda for the incoming government.