A new national audit report says Customs is missing much of the contraband, including drugs and guns, arriving in international mail.

Close to half a million prohibited items could have evaded Customs mail screening in 2012-13, allowing guns, drugs and quarantine risk items into the community.

The Australian National Audit Office (ANAO) says Customs’ seizure of 67,123 prohibited imports represented around 13 per cent of potential prohibited imports.

In a 2012 report on its own performance, Customs cited the increasing number of successful seizures of contraband and concluded that this demonstrated “low level of leakage of a small number of prohibited imports of a minor nature.”

But the ANAO said Customs really didn’t know as it had never calculated its leakage rate – done by sampling cleared mail items.

ANAO did its own calculations and estimated Customs missed 467,893 prohibited items in 2012-13.

“Customs’ high estimated leakage rate, particularly in unscreened non-letter class mail would suggest that Customs’ screening activities miss a large number of prohibited imports,” it said.

ANAO acknowledged screening was a challenging job.

In 2012-13, Australia Post reported that around 180 million international mail items arrived in Australia. Over the last five years, parcels have increased 200 per cent with the rise in online shopping.

Most pose no threat but some contain drugs, guns and risky plant or animal products.

International mail arrives at four Australia Post gateway facilities in Brisbane, Sydney, Melbourne and Perth where it’s screened by Customs and Agriculture visually and by x-ray scanners and sniffer dogs.

Where previously everything was screened, it’s now done according to risk assessment.

Since 2006-07 the number of items screened by Agriculture has reduced by 76 per cent and 45 per cent for Customs. The government provided funding this year to increase the screening rate.

ANAO said Customs described its targeting approach as “intelligence-led and risk-based”.

But in practice targeting decisions were often not documented and did not align with risk analysis, providing little assurance that it was adequately and consistently targeting high risk mail groups.

Customs and the Department of Agriculture have agreed to ANAO recommendations to improve screening procedures.