Our navy had to knock back almost half of Customs' requests between August 2015 and September 2016 for help checking out suspicious small craft because the aircraft and ships needed were being replaced, repaired or undergoing maintenance.
Two of the refusals from the NZ Defence Force came during the month that half a ton of methamphetamine was found on Ninety Mile Beach.
But Defence Minister Gerry Brownlee said that the vessel on which the methamphetamine was transported to New Zealand was not one of those the navy had been asked to inspect.
The amount of methamphetamine found being shipped into New Zealand is believed to be dramatically on the rise with drug syndicates shifting to importing the drug rather than manufacturing it here.
Information sought by the Herald through the Official Information Act shows there were 25 requests for help by NZ Customs between August 2015 and September 2016 of which the Navy was able to assist on only 14 occasions.
The number could have been even higher but Customs did not seek NZDF help checking small craft for at least seven months before August 2015.
Navy chief of staff Commodore Ross Smith said the numbers were skewed because requests for help came through a period during which the Sea Sprite helicopter was being replaced with a newer version.
"The NZDF would ordinarily have a Seasprite helicopter available at short notice for taskings of this nature," said Commodore Smith.
He said "pilot and crew resources were prioritised to bring the new Seasprite into service".
The OIA response shows the inability to help because the new Seasprites were due ruled out naval helicopter support for 11 months.
He said other missions were at times given priority, such as the P-3K2 Orion being used for a search and rescue mission.
He said the defence force's "comparatively small fleets of vessels and aircraft" were needed across a range of military and non-military tasks and also needed maintenance.
"Managing all of these competing demands is the challenge our teams of operators, maintainers and planners grapple with on a daily basis."
In August 2015, on its first request after the lengthy gap, Customs asked the Navy to send a Seasprite helicopter to investigate a small craft about which it had suspicions.
Even though it was the first request for more than half a year, the navy had to say 'no' because the old Seasprites couldn't be used and the new helicopters were not yet available.
It managed to send a ship to meet the navy's single request the following month but in October knocked back Customs on three occasions.
The reasons cited in the 11 instances assistance wasn't able to be offered were the replacement of the Seasprites, planned and unplanned maintenance and repairs to aircraft or ships.
A Customs spokeswoman said there were "a number of mechanisms in place to ensure New Zealand's coastline is not used to smuggle illicit drugs".
"The use of Defence Force assets is only one layer of defence."
She said if aircraft or ships could not be sent to check incoming vessels then Customs turned to its Coastwatch network and land and sea surveillance.
"The methamphetamine shipment to Ahipara was not a request NZDF was unable to meet."
At least seven people have been arrested and charged after the discovery of the near 500kg of methamphetamine near Ahipara in June. The confiscated drugs have an estimated street value of half a billion dollars.
The bust was made after locals reported suspicious activity from boaties and also a low flying twin-engined plane in the area.
While she confirmed Customs did not ask NZDF for help for seven months, she did not say why.
She said Customs sought help "based on our intelligence and risk management approach" and also could use other options to watch the coastline, including its own vessel Hawk IV.
Brownlee said he was satisfied NZDF had enough resources to play its part and believed it had done so.
But Labour's defence spokesman Ian Lees-Galloway said the seven-month gap raised the question as to whether Customs didn't ask because it knew what the response would be.
"I would have thought patrolling our coastline and supporting Customs should be one of the core tasks for the defence force - particularly navy."
Labour has previously raised concerns over inshore patrol vessels - such as those Customs request.
Figures showed two of the vessels had not been to sea for years and others had been tied up for months because they did not have enough people to crew the ships.