As Trans-Tasman Resources (TTR) outlined why it wants to vacuum the Taranaki seabed for millions of tonnes of iron sand, opponents held a "silent protest" calling for a moratorium on all seabed mining.
The company says the project would see it spend $150 million a year in New Zealand.
But opponents say there would be little financial benefit for taxpayers and are calling for a moratorium on iron sand seabed mining which they say could stretch from Whanganui to Northland.
The Environmental Protection Authority hearing started in Wellington yesterday to consider the first marine consent application under new laws to mine iron sand from a 65.76 square kilometre section of the South Taranaki Bight.
TTR, about 90 per cent foreign-owned, has already spent more than $50m on the project since it was established in 2007 to explore, assess and develop the iron sands off the North Island's west coast.
The judgment from a five-strong panel would set a precedent for future seabed mining applications. TTR chief executive Tim Crossley started proceedings with a general outline of the application which critics say would potentially harm the marine environment.
The company has applied to vacuum 50 million tonnes from the seabed a year, for 20 years.
It had already spent "well in excess of $8 million" on research focused on the environment and the effects the mining proposal may have on it. "We would say no other past marine project, other than perhaps investigations into the Maui gas fields in the 70s and 80s, has been researched as deeply."
The process would see iron sand vacuumed onto a vessel where magnetite concentrate would be extracted, then pumped to a second vessel to be cleaned using fresh water. It would then be transferred to another ship to be transported to world markets.
The system allows for "single pass" mining - the seabed is vacuumed once and then backfilled - on 300 metre by 300m predetermined blocks. The submarine crawler which vacuums the sand would move at between 60m and 70m an hour.
"In reality our environmental footprint is confined pretty much to a 300m by 300m space" at any one time, Crossley said.
Every item on the production chain would utilise proven "mature technology", he said.
If mining were to start, TTR would spend around $150m a year, benefiting New Zealand's economy in general and Taranaki's in particular.
"We are extremely confident that the science and economic analysis is robust."
Kiwis Against Seabed Mining (KASM) used Facebook to organise a silent protest outside the hearing. Chairman Phil McCabe said before the hearing that the precedent-setting nature of the case, and inadequate evidence provided by TTR that seabed mining would not damage the environment, made a good case for a moratorium.
"The west coast from Whanganui to Cape Reinga is up for exploration.
"If this Government gets what they want you'll see these types of operations happening throughout our waters."
He said moratoria on seabed mining were already in place in Namibia and Australia's Northern Territory.
Protester and Green MP Gareth Hughes also called for a moratorium "across all the seabed mining projects" proposed for New Zealand.
"It really is the frontier of resource extraction and we are a world first trial bed. We need to make sure we aren't damaging the environment before we undertake projects such as this."
The hearing is scheduled to run till early May.