An early guilty plea is expected from the operators of Taranaki's Tui oil field after 23,000 litres of oil spilled last October, coating more than 10km of coastline southwest of New Plymouth.
Maritime New Zealand has charged Tui's shareholding operator Australian Worldwide Exploration, and Prosafe, which runs offshore production.
Each company could be fined up to $200,000.
AWE New Zealand manager Dennis Washer said an early guilty plea was likely.
The operators had since focused on preventing further spills, with stricter processes for the production ship and extra training for workers on board, he told Radio New Zealand.
AWE will pay for the $100,000 clean-up by the Taranaki Regional Council.
The Tui field is producing about $7 million worth oil a day.