New Zealand Refining chief executive Sjoerd Post said the shutdown was an opportunity to perform process cleaning, maintenance and capital project work that could not be carried out on the run.
On average 300 people work at the refinery but with the maintenance work that will swell to 1000 workers every day.
A major piece of equipment arrived on Tuesday from Melbourne. The cylindrical column stands about 5.5m in height and 7m in diameter and is used in a vacuum unit.
Post said the current vacuum unit, installed in the 1980s, had started to show some corrosion on the inside. The 25,000 tonne unit was unloaded from the ship, placed on a truck and would be craned into position.
Chris Jurlina, from Refining NZ, said the shutdown allowed the teams to asses the equipment and look at the integrity of the equipment.
The vacuum unit walls were 18mm thick and made of carbon steel with a 3mm stainless steel layer on the inside.
"The stainless protects the carbon steel."
Jurlina estimated it would take a month to install the unit.
"This part of the project is the critical path project and the one that is the longest."
The piece brought in this week was the central part of a 34m high vacuum column. The top of the column would be cut off and lowered to the ground, the existing middle section would then be cut out and removed before the new unit is put in its place and the top reconnected.
The shutdown is expected to cost about $85m, with much of that money going into the Whangarei community, through workers, contractors, equipment and accommodation.
Bream Bay accommodation providers reported extra people staying who were working on the shutdown.
One Tree Pt Motel owner Kate Neal said the motel got the benefit of extra visitors every time there was a shutdown at the refinery.
''The refinery is very supportive of local businesses and we certainly get some benefit [from the shutdown].''
