The New Zealand Refining Company in Northland on Monday warned of dire consequences if power supply to the region is lost because of Auckland's frail network.
The Electricity Commission has been hearing submissions after it opposed a $480 million proposal put forward by national grid operator Transpower.
At present, one double circuit line from South Auckland services the entire area from North Shore to Cape Reinga. It has been identified as being at extreme risk of faults.
The plan would create a second power route north of Auckland through a series of tunnels.
New Zealand refining company chief executive Ken Rivers says the country would grind to a halt if there was a fault in the current sole source of power north from Auckland.
Mr Rivers says electricity stocks would fall during a prolonged outage and the country would shut down because there would be not petrol, diesel or aviation fuel.
Transpower told the commission on Monday that electricity supply to customers north of Auckland is at risk of a substantial outage and could take up to 12 hours to restore.
Transpower, together with business and energy sector leaders want the proposal to go ahead.
In December, the Electricity Commission signalled it intended to reject the project, saying there would be better cheaper options.
On Monday, it said it would take some weeks to reach a final decision.