The country's only oil refinery at Marsden Pt has is shut for two weeks while its undertakes planned maintenance.

Refining NZ said units have shut down at Marsden Pt for planned maintenance on the Refinery's Hydrocracker, which began on Saturday.

The two-week pit-stop for a Hydrocracker "top-bed-skim" will allow for maintenance, inspection and repair of equipment that is normally in operation.

Refining NZ chief operating officer, Andrew Brewer said that the planned maintenance on the Hydrocracker was undertaken every other year and was critical for process safety.

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"A key part of the top-bed-skim is refreshing the catalyst package in the top bed of the Hydrocracker unit. This keeps the unit running safely and will allow the rest of the catalyst in the Hydrocracker to achieve a three to four-year life."

The planned maintenance is forecast to cost around $2 million with a large proportion of that going directly to the Northland community through payments for employment, contracting, equipment, accommodation and other services.

"Our team has been preparing for over 12 months to ensure the safety of everyone working on site, as well those in our community. For residents the most visible part of this pitstop will be flaring, which allows refinery gases to be vented safely as units are either depressurised or brought back into operation," Brewer said.

"With the preparation done and dusted, we are on track to deliver a successful top-bed-skim, safely, on time and to a high standard with no impact on the environment."

Further information including progress reports, will be available via the company's Facebook page.