Mobil Oil New Zealand has been fined $288,000 for a major heavy oil spill into Tauranga Harbour .

The cause of the discharge was oil escaping from two holes in a rusted lateral pipe which was a key part of the Mobil's bunkering (ship refuel) pipeline system.

The company was sentenced in Tauranga District Court today after earlier pleaded guilty to charge of discharging a contaminant into water.

The court heard Mobil failed to take crucial repairs when a corroded pipe was first identified years ago.

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The charge attracts a maximum penalty of $600,000.

Between 3000 and 4000 litres of heavy fuel leaked from a lateral pipe into the harbour on April 27, 2015.

The spill initially affected Tauranga Bridge Marina, and the travelled in the harbour where it washed ashore at Maungtapu Bay, and also reached Pilot Bay, Mount Maunganui and Matakana Island.

Mobil has paid $1.8 million in reimbursements and other costs including reimbursing the regional council almost $1.2 million for costs incurred in the clean up.