Environmentalists in Taranaki are calling for Shell Todd to come clean on the chemicals it plans to use to extend the life of the field.
The company told the Environmental Protection Authority it wanted to withhold elements of its Discharge Management Plan from the public.
It said the information about specific chemicals used to drill and complete wells included "trade secrets".
But the EPA said in the past other operators had made such plans available for supervised viewing by other submitters and the decision-making committee.
It was seeking Shell Todd's view on adopting the same approach.
Catherine Cheung from the group Climate Justice Taranaki said it wanted to know exactly what would be discharged at sea, and as a submitter, should be able to examine the plan.
"There will be critical information that explains what chemicals they are using and what may be discharged into the ocean and on the sea floor. And we feel like there's too much secrecy in there and it's not a transparent process."
Shell Todd said it had submitted more than 300 pages of detailed information that would be considered during the EPA hearings this month.