Refining NZ has made a net loss of $6.9 million for the June half year, a reversal from the $5.2m profit it made in the same period last year.
Total income was $92.4m, down 27 per cent from $126.4m previously.
The company will not pay a half-year dividend.
The company said it had had to weather a difficult start to the year with weaker refiners' margins and a high New Zealand dollar.
Company chairman David Jackson said the loss for the half year was to be expected given the harsh conditions, with a marked decline in Singapore refiners' margins.
The gross refiners' margins in the half year was US$1.66 (NZ$1.97) a barrel, down from US$5.27 in the previous half year.
Singapore complex margins fell to a low of only US10 cents a barrel in May and June.
Refining NZ invoked a fee floor for processing for customers, with a total of $36m paid in the first half.
The result had also been influenced by the high New Zealand dollar, averaging US84c in the first six months, which knocked back processing fee revenues.
The company's $365m Te Mahi Hou project was progressing to plan, with $240m spent so far.