An extra 250 people will be working on a three-week shutdown and planned maintenance programme starting soon at the Marsden Pt oil refinery.
Flares that will shoot out of the stacks during the process, which will start on Saturday May 2, are part of the safety measures.
General manager Peter Gubb said the refinery had a dedicated Health, Safety and Environment plan to ensure the shutdown proceeds safely and without impacting the local community or the environment.
"The flaring that will be visible when we're shutting down and starting up process units is a key safeguarding process that allows hydrocarbons to be vented when units are being de-pressurised," he said.
The shutdown for regeneration of catalyst on the Platformer unit and other essential maintenance has required detailed planning and extra resources, including 250 more people on site.
The majority of the workers are from local contracting companies working at the refinery.
Mr Gubb said the planned maintenance on the Platformer was essential for the refinery's ongoing reliability and safety.
"The shutdown has been scheduled into product supply plans and agreements with the refinery's oil company customers to ensure that product availability isn't affected.
"This will be the last planned maintenance shutdown on the Platformer before it's replaced by the CCR unit, expected to be up and running when the Te Mahi Hou project completes early to mid-November."
Te Mahi Hou Project replaces Refining NZ's existing petrol making plant that has been in operation for around 50 years and which would otherwise require an investment of approximately $105 million to extend its operational life.