About 350,000 workers across the country held an active security clearance last month, more than enough for every public servant, military personnel and defence contractor.

Even the government is unaware of the extent to which the use of clearances has increased. Until recent years, each workplace organised its own vetting, and neither the total number of clearances nor the public cost of vetting was ever reported.

However, 15 years ago, only about 15,000 staff were vetted each year, suggesting successive governments had abandoned the long-held policy of giving clearances to as few people as possible.

Government staff need a security clearance if they must access classified documents to do their job.

Yet the latest employment gazette suggests the requirement – which involves expensive background checks – is now applied unthinkingly.

The Industry Department in Canberra is hiring a plumber, who will need a clearance. The Plague Locust Commission wants two junior staff (APS level 2 officers) for field work in country NSW and Queensland. They, too, must be vetted.

Parks Australia is employing a manager for its tourism centre at Jabiru in Kakadu National Park. It is also seeking a taxonomy adviser to update a database of plant names. Both jobs require security vetting.

The issue has drawn the attention of the public service watchdog, the Auditor-General's office, which is investigating the Australian Government Security Vetting Agency's effectiveness.

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The Gillard government was so concerned by the soaring costs of security checks that it set up the agency in October 2010 to eliminate wasteful vetting and duplication, saying a centralised service should save taxpayers at least $5.3 million a year.

But the Defence Department will not say how much these background checks now cost, as the budgets for security and intelligence work are not made public.

The Australian Law Reform Commission's 2010 report on secrecy laws warned the government that over-classifying information, or failing to declassify it, "could prevent information sharing for the purpose of whole-of-government initiatives".

The Auditor-General found previously that public servants tended to classify documents that should be public or gave them unnecessarily high classifications, which had "the effect of increasing the costs of protection and restricting the flow of information".

Former Customs official Peter Bennett, president of the National Whistleblowing Information Centre, said the bureaucracy's use of security classifications had "spiralled out of control to ridiculous levels".

He said his former boss, former Customs head Lionel Woodward, told staff to classify all information created in the agency as confidential.

"If someone wrote down their grandmother's scone recipe, it was a classified document. That's how ridiculous it was," he said.

"If you have a look at the type of information you're entitled to access from the US government, then look at what you're entitled to access from the Australian Public Service, you begin to see that we're as closed as China."

Mr Bennett said an equally significant problem was the failure to declassify documents after they lost their sensitivity.

"When you classify information, you need to also ask 'how long should this remain classified?'. Will it be sensitive for six weeks? Six months? Yet very few public servants even think about that," he said.

"That's where the costs start to mount, and all of a sudden every public servant needs a security clearance because everything's classified."

with Michael Inman