Mr Mark said it was particularly concerning considering the Government wants to add a further 620,000sq km to the Navy's patrol zone with the creation of the Kermadec Ocean Sanctuary.
"Given we're responsible for a vast slice of the globe that extends from Antarctica to north of the equator and mid-Tasman to east of the Cook Islands, our Navy is frankly way too small and our seas are mostly unguarded."
But Defence Minister Gerry Brownlee, who said the Navy's quite capable of putting a full flotilla to sea, blamed the last Labour government for ordering too many in-shore patrol vessels and not enough off-shore vessels like the HMNZS Otago and Wellington.
"It's an extraordinary situation. I cannot believe the previous Government actually committed to that number without some idea if they'd use them. What we could do with more of is off-shore patrol boats," he told Newshub.
"You'll have to go back to the Foreign Minister of the day and ask him why we were purchasing them at the time."
He defended the lack of sea days clocked up, and said they're busy with scheduled maintenance or other exercises.
"They're not going to sea for very good reasons. You have a number of days at sea, and then a number of days in port, probably a greater number of days, when maintenance is being done or crews are in some other form of training."
Labour's defence spokesman Phil Goff said that explanation was deliberately misleading.
"The inshore patrol fleet was commissioned after a 2001 Maritime Forces review said that the vessels were needed for fisheries protection, border control, conservation support, and search and rescue. These needs still exist.
"Year after year Parliament's defence select committee has been told the Navy can't put many of its inshore patrol vessels to sea because it doesn't have skilled personnel to staff them.
"The problem goes back to 2012 when National's civilianisation programme saw an unprecedented attrition in the Navy of 23 per cent. It takes years to replace skilled personnel."
