The Maritime Union said it is important the country's two fuel tanker ships are retained to ensure energy security when the Marsden Point refinery closes.
It said the proposed change of the refinery from a processing to an import storage facility raises questions as to how fuel will be delivered to New Zealand ports.
The New Zealand flagged ships, the MT Kokako and the MT Matuku, are owned by Silver Fern Shipping Limited and chartered to Coastal Oil Logistics Limited (a joint venture by BP, Z Energy and Mobil), which transport fuel products from Marsden Point to regional ports.
The union's national secretary Craig Harrison said the ships will be useful in contingencies.
"We've had tankers on the coast since 1927 in New Zealand, and they've been running around our coast without any incident for a long time, supplying our regional ports with fuel.
"If you cast back to a couple of years ago when we had the ruptured pipe out of Marsden Point to Auckland and the effects it had on fuel supply to Auckland, we were relying on ships to bring it down from Marsden Point and basically multiple trucks," Harrison said.
He said without those ships, the country would have to rely on foreign vessels, which can always bypass New Zealand.
"Global shipping and logistics are already in turmoil, with major problems due to shipping shortages and port congestion.
"If fuel deliveries were to come in on overseas ships direct to New Zealand ports, this leaves New Zealand wide open to the type of disruption that has affected shipping and ports globally in the last year, but for essential fuel supplies," Harrison said.
Coastal Oil Logistics Limited chief executive Jon Kelly said the company was working with shareholders and key service delivery providers to evaluate the future of its operations following Refining NZ's decision.
"Refining NZ's plans, once a final decision is made, are expected to have a significant impact to COLL's future operations, and COLL is working to understand the full extent of these changes.
"In the meantime, COLL remains firmly focused on working with industry participants, including Silver Fern Shipping Limited, who COLL time charters its two MR tankers from, to deliver a safe and efficient shipping service for petroleum products on the New Zealand coast," Kelly said.