The amount of oil contained in the Tui field is less than initially thought.
An assessment of the field has cut reserves by 10% to 15% to about 40 to 42 million barrels.
The Tui field was previously reported as having 50.5 million barrels.
AWE, the field's operator, will carry out more work to confirm the level of reserves.
AWE managing director Bruce Clement says the operator will now carry out more work to confirm the new level of reserves.
Mr Clement says he's confident further exploration of the offshore Taranaki field will find more oil.
The field has produced about 31 million barrels of oil since it was brought on stream.
New Zealand Oil & Gas, which owns 12.5% of the field, says the cut is disappointing. It says if confirmed it that would represent a reduction of 1.1 million to 1.3 million barrels to the company.
Shares in New Zealand Oil & Gas fell 11 cents or 13% to 73 cents on Monday, while another owner, Pan Pacific Petroleum, fell 1.5 cents to 16 cents.