POLICE suspect about 25 major shipments of illegal firearms destined for outlaw bikie gangs and Middle Eastern organised crime groups slipped past Customs checks in the past two years.

An 88kg shipment of ice, heroin and opium is also believed to have been missed by Melbourne Customs officers despite the pallet they were hidden on being X-rayed.

The shipments by air and sea are revealed in "missed detection" reports obtained from the Australian Customs and Border Protection Service. Missed detections are reported when shipments of guns, drugs and other contraband are detected after passing through Customs, usually by police.

The missed firearms deliveries included a "large cache" of up to 220 Glock and Taurus handguns, imported on forged papers in up to 20 shipments of parts from Germany, and five suspected imports of illegal firearms from Nashville hidden in motor vehicle engines.

Police say the highly organised importation of guns in parts was discovered after several weapons, including a Glock handgun, were seized during investigations into a string of armed robberies, including one where shots were fired in a busy street during a bungled attempt to rob a cash-in-transit van last year.

Operation Otford investigators identified six consignments of interest imported since August 2011. One consignment was targeted and referred for examination but no weapons were found and the remaining five shipments were not checked. Police believe at least one of those shipments was a missed detection and the remaining four shipments are considered "highly suspect".

The missed shipment of drugs - 40kg of ice, 2kg of heroin and 46kg of an opium substance - was discovered by NSW police in Operation Taipan in February last year.