National On Scene Commander Rob Service said the team at the incident command centre was modelling where the estimated five to 10 tonnes that leaked from the ship on Saturday night was heading.
The oil was being broken up naturally in the water and was likely to reach shore in the form of patties or tar balls, he said.
On the current projections, the oil could reach the shores of Mayor Island or the Coromandel by the end of the week, he said.
MNZ is preparing contingency plans if the substance reaches shore, and operational teams will assess response options.
But there was good news this morning (Wednesday) after MNZ revealed more than half the fuel on board after salvors worked through the night.
By 3pm yesterday, salvors had removed 645 tonnes of fuel oil of the estimated 1700 on the ship when it grounded.
Salvors estimated they passed the halfway point of 675 tonnes overnight, and the figure will be updated this afternoon.
An estimated 350 tonnes has spilled from the ship.
MNZ Salvage Unit Manager Bruce Anderson warned the second half of the fuel posed some serious challenges as the oil was spread across the tanks.
"The salvors now have the pumping system working well and they are getting good transfer rates, which is excellent.
"However, the second half of the oil is in around four tanks, rather than one _ and one of them is submerged underwater.''
Mr Anderson said the salvors would be working today to establish a fuel transfer system from the three tanks in the engine room, which hold a combined 250 tonnes of oil. The oil would be trasferred to a tug which could speed up the process.
