More than 20,000 requests for historical documents with the National Archives of Australia are awaiting a response with more than half of those submitted more than five years ago, a recently-answered question on notice has revealed.

Nearly 200 of the retrieval requests were for documents relating to the Australian Security and Intelligence Organisation, of which more than half were lodged five years ago or more.

Requests for documents are to be notified of a decision by the archives within 90 days of lodging the application under legislation.

The domestic spy agency defended its delays, pointing to the complex task of releasing sensitive documents along with the agency's various other competing priorities.

"ASIO works closely with the National Archives of Australia to facilitate the release of archival records in response to requests, while balancing various and often competing priorities," a spokesperson said.

"The release of archival records necessarily involves rigorous and time-consuming assessments to ensure the material does not damage Australia's security or put our people and capabilities at risk."

It added the agency categorised requests into "fast track" or "bulk access".

The former were usually family-related requests or requests that didn't require intensive resources from the agency. Bulk access requests, however, were determined by a number of factors and could range to searching for non-existent documents to a search covering thousands of pages.

But independent senator Rex Patrick, who asked the question during May's Senate estimates, said the growing backlog, including from within various agencies, demonstrated a clear failure by the government to adequately fund the archives agency.

"Nearly a decade after his appointment, National Archives Director-General David Fricker has conspicuously failed to deliver on a core responsibility of his agency - providing timely public access to past government decision-making," Senator Patrick said.

"That said, it is not all the Director-General's fault. At the end of the day it is the Prime Minister and Cabinet who decide the resources available to [the National Archives].

"Prime Minister Scott Morrison is happy spending half a billion dollars on a massive expansion of the Australian War Memorial in Canberra, but appears equally content to see much of Australia's history sit at the bottom of a memory hole, inaccessible to historians, journalists and other researchers."

The figures come less than a month after the federal government announced it would inject $67.7 million into the archives to address concerns many historical documents were at risk of deterioration before being digitised.

A spokesperson for the National Archives confirmed a portion of the funding will go towards additional staffing and resources for its document retrieval functions.

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National security expert at the Australian National University Professor John Blaxland said the long delays are a result of a number of factors, including the unexpected surge in popularity of the archives along with funding shortages.

"What we have seen is an explosion in interest in research about families, about lineage, about heritage, and about issues of national security among many other things, that has butted up against the funding realities," Professor Blaxland said.

"And the funding realities are that the National Archives has not been resourced to match the legislative mandate it has been given.

"It's deeply unsettling. We shouldn't have this backlog but the combination of factors has just blown things out of the water."

Part of the delay was that the process, especially when it related to intelligence agency documents, was a complex and cumbersome task.

Ensuring there were consistent redaction decisions across related documents was critical, he said.

"If you redact one document well, and another document not well and you then correlate them, the redactions can negate each other," Professor Blaxland said.

"To manage the process of redactions well between documents and between files requires a level of expertise that is rare and it's hard to find and maintain."

In an era where foreign interference remained a high risk, and documents were available immediately online once released, it was particularly important to ensure reviewers were careful, he added.

"There are some pretty darn closed societies out there who are quite happy to exploit our openness and use it as a weapon against us," he said.

"It means you've got to really tread pretty carefully."