The New Zealand Refining Company says its margins remained healthy in March.
Average gross refinery margin slipped to $US5.55 per barrel in March and April from $US6.85 in January and February.
Processing fee income was $37.1 million, on 6.8 million barrels produced.
The company says some refining equipment was shut down in April to replace a part and for scheduled maintenance and inspections.
It says the shutdown should run until the end of May, and reduce intake by about 20% for the month.
Brent crude oil on Tuesday was trading at $US74.85 per barrel.
