Australian Agriculture Department officials have estimated that nearly half of all imported raw prawns were infected with white spot disease before imports were suspended in January.

The officials told a Senate Estimates hearing in Canberra on Wednesday that six importers supplied "the vast majority" of the contaminated prawns.

The Department has since revoked their import permits and approvals.

"We have made a referral to the CDPP [Commonwealth Director of Public Prosecutions], to pursue charges and further referrals are being considered," the Department's head of biosecurity, Lyn O'Connell, told Estimates.

Contaminated raw prawn imports have been blamed for an outbreak of white spot in Queensland's Logan River and Moreton Bay.

Logan River prawn farms will remain shut down for one year, in an effort to contain the outbreak.

While departmental officials said the source of the outbreak may never be known, they are still working on the assumption the virus can be eradicated.

Testing failed detection

LNP Senator Barry O'Sullivan said the outbreak "represents a catastrophic collapse" of Australia's biosecurity arrangements and the so-called risk-based approach to raw prawn imports which involved testing a percentage of prawns at the border.

Agriculture Department investigations revealed that a number of importers engaged in the "deliberate subversion" of those processes, including deliberately mislabelling prawns as other product to avoid testing.

Investigations also revealed that white spot testing at the border failed to properly detect infected prawns.

Under more sensitive testing at the Australian Animal Health Laboratory, a large percentage of raw prawns that were initially cleared at the border were subsequently found to be infected with white spot.

"The estimation that we have is that nearly half of the prawns imported into Australia were positive for white spot syndrome," departmental official Tim Chapman told the senators.

"Obviously it is unacceptable to us."

Agriculture Minister Barnaby Joyce announced in May that the Commonwealth Government would allocate $20 million to prawn farmers in the Logan River, who must cease farming for 12 months.

Some of that will be paid back by the prawn farming industry through an agreed levy on production.

But Labor's shadow agriculture spokesman Joel Fitzgibbon accused Mr Joyce of being "missing in action" on the issue.

"He saw political opportunity in Johnny Depp's dogs, an issue that was already being adequately handled by the department, but when it comes to this serious biosecurity breach which is so seriously impacting on prawn farmers in this country, Barnaby Joyce is nowhere to be seen," he told Parliament.