In its December half-yearly interim report released late last week, NZOG said that the costs of production per barrel of Tui oil had been substantially below the forecast of $US10 ($A10.90) per barrel.

This and better than anticipated oil flows - the 10th million barrel was produced a week ago - enabled the partners, headed by operator Australian Worldwide Exploration, to achieve the recovery of all exploration and development costs by mid-December, NZOG said.

NZOG said the capital cost of bringing the Tui, Amokura and Pateke oil pools into production was US$274 million ($A298 million).

The partners' leasing of the Tui floating production, storage and offloading vessel Umuroa, on a fixed five-year charter, was costing $US135 million ($A147 million), with five one-year options to extend the charter.

NZOG also said the Tui project was not only a great commercial success, but that it had many noteworthy technical achievements.

These included geosteerable drilling of the four horizontal production wells (a first for New Zealand), the longest horizontally drilled wells in New Zealand, subsea wellhead completions (the first of their kind in New Zealand), and modifications to the Umuroa that included a swivel production turret, allowing the vessel to swivel for currents and weather while maintaining production output.

NZOG recorded a net surplus of $NZ41.4 million ($A36.2 million) - from total revenue of $NZ95.5 million ($A83.5 million) - for the December half-year.

That revenue figure included $NZ84.2 million ($A73.62 million) from Tui oil and a gain of $NZ11.2 million ($A9.79 million) resulting from the successful float of associate company Pike River Coal.

NZOG said these latest figures compared with a net surplus of just $NZ500,000 ($A437,200) and revenue of only $NZ92,000 (($A80,445) for the corresponding 2006 period.

The company added that it had about $NZ85 million ($A74.32 million) of accumulated tax losses available for use in the June 2007-08 year and expected all these losses to be used.