New Zealand Refining chief executive Sjoerd Post will leave the Marsden Point refinery operator at the end of July after five years in the role.

The Whangarei-based company announced Post's decision, saying the five months until his departure provides time to recruit a new CEO and allow for an orderly transition.

Post came on board at the start of 2013, coinciding with a squeeze in global refining margins.

Under his watch, the company completed its $365 million Te Mahi Hou upgrade and extension to boost production, and more recently dealt with the outages on the refinery-to-Auckland pipeline.

No one will be prosected over the pipeline rupture that sparked a major fuel shortage last year, mainly because authorities do not know who did it.

The pipeline break disrupted supplies from the Northland refinery and caused dozens of flights to and from Auckland Airport to be cancelled.

Two digger drivers were interviewed about working at the Ruakaka site when Refining NZ's Auckland fuel products pipeline was ruptured, but neither could recall working near the pipeline.

Northland Regional Council group manager of regulatory services Colin Dall said the fuel leak - estimated at 124 cubic metres – was a breach of the Resource Management Act but the council "does not have a case for prosecution".

Dall said lengthy investigations found an unknown digger illegally searching for swamp kauri "may have" operated around the pipeline at the Ruakaka property.

Gouges apparently caused by a digger were believed to have triggered the pressurised pipeline's failure, but the actual date of the damage and what did it were not known.

The damage could have happened any time after the pipeline's last inspection with a "magnetic flux leakage intelligent and sizing calliper pig" in July 2014, he said.

The company's share price has declined 6.3 per cent, or 16c, since then, recently trading at $2.38, and has paid 47c per share in dividends, not including whatever it declares in tomorrow's annual earnings result.

"Sjoerd has made an outstanding contribution over the length of his tenure, building a culture that is now strongly evident in the performance and resilience of the business," chair Simon Allen said in a statement.

"The board is pleased that Sjoerd has confirmed he will remain in the business for the next few months, seeing us through the succession process and a number of critical work programmes - including our impending refinery shutdown and the government inquiry into the pipeline incident."

Forsyth Barr analyst Andrew Harvey-Green forecasts NZ Refining will report a 73 per cent gain in annual profit to $81.6m tomorrow on a 16 per cent increase in revenue to $410.6m.

He also predicts the board will declare a final dividend of 8c per share, up from 6c a year earlier.

- BusinessDesk, Northern Advocate