The $85 million maintenance shutdown of the Marsden Point refinery is about to enter its busiest phase with work on refinery processing units and related utilities starting next week.

Scheduled for completion in mid-June, the shutdown will see staff and contractor numbers at the refinery swell from 500-plus to around 1200.

The extra personnel cover a broad range of expertise including engineering and mechanical, labouring, rigging and scaffolding, water blasting, safety, crane driving and supervisory.

A large portion of the $85 million cost is going directly into Northland businesses through payments for employment, contracting, equipment, accommodation and other services.

The shutdown allows statutory inspections of equipment across the refinery, along with maintenance work, cleaning and other projects that cannot be carried out while in operation.

A critical part of the shutdown will be the 10-day closure of the utilities systems for preventative maintenance.

Refining NZ CEO Sjoerd Post said preparation for the shutdown had gone well.

"We're pleased with the progress made to this point and grateful for the efforts of everyone involved."

"This is the first shutdown in 14 years where the entire refinery will be shut for a short period of time," Post said.

"This is a substantial project, with a number of major pieces of equipment being replaced. To give some idea of the scale of the work involved we have around 2000 tonnes of scaffolding installed in the refinery."

An estimated 1200 staff would be on site by mid-May and would come from all over the world.

Post said a Health, Safety and Environment plan was an essential part of the preparation and as a safeguard, there would be flaring when shutting down and starting up process units to allow hydrocarbons to be vented safely as units are de-pressurised.

The shutdown had been scheduled into product supply plans and agreements with the refinery's oil company customers to ensure that product availability was not affected.