More than half a tonne of methamphetamine has been seized in one of New Zealand’s largest importations of the drug, concealed in steel beams.

Two men visiting from Australia being arrested in the process of deconstructing the beams after a joint investigation by New Zealand’s Customs and police.

Customs earlier located the concealment of 515 kilograms of methamphetamine within the hefty steel beams in early September, after being sent from the United States.

Customs’ intelligence gathering and associated targeting prompted further investigation of the shipment, which uncovered inconsistencies in some of the 42 steel beams.

Customs’ Sea Cargo Inspections Facility officers x-rayed selected items in the consignment and conducted other additional examinations which led to the discovery of the large amount of methamphetamine.

All up five men, aged between 31 and 51, have been arrested. All initially appeared in the Auckland District Court on Friday, facing charges relating to the importation and supply of methamphetamine, according to New Zealand’s Customs service.

One of these men has been charged with unlawful possession of firearms after a military-style semi-automatic rifle and ammunition were recovered, the Customs said.

The two men arrested in the Waikato on 4 October are New Zealand passport holders but reside in Australia.

Investigations will continue and further arrests cannot be ruled out.

Detective Inspector Colin Parmenter, from the National Organised Crime Group, says the seizure is a significant disruption to the wider drug market operating within New Zealand.

“It’s estimated that this shipment would have gone on to produce 25.7 million individual doses of this destructive drug, and preventing this harm is a key motivation for our staff.

“The organised criminal groups looking to profit from this type of offending will continue to be a key focus of our attention and resources.”