The inquiry into Labor’s franking credits policy is struggling to cope with an inundation of submissions, one in five of which contain text written by its own chair, the Liberal MP Tim Wilson.

The controversial inquiry is due to finalise its report on Monday, and Wilson, as head of the House of Representatives economics committee, is currently preparing the chair’s draft.

But Wilson finds himself in an unusual position. At least 249 of the 1,300 submissions he considers will contain text that he wrote himself attacking Labor’s policy, according to a Guardian analysis.

The inquiry will also not be able to consider all submissions made by the public, because the sheer volume has meant they could not all be processed in time. Labor has described that situation as “farcical” and “completely unacceptable”.

Many of the submissions are being pushed to the inquiry from a campaign website that Wilson created and authorised, stoptheretirementtax.com, which allows individuals to send in a pro forma submission opposing Labor’s policy.

The website is partly funded by the fund manager Geoff Wilson, an opponent of Labor’s policy and a distant relative of Tim Wilson. Tim Wilson also owns shares in investment companies managed by Geoff Wilson’s firm, Wilson Asset Management.

A Guardian analysis last month found the website’s pro forma text submissions accounted for at least 10% of those given to the inquiry. That has increased to 20% in little more than a month as submissions from Wilson’s website continue to stream in.

The sheer volume of material is creating problems for the secretariat, which is required to process submissions before they can be uploaded and viewed publicly.

A spokesman for the committee secretariat said staff were working through a “significant backlog of emails and submissions”.

He said the workload was so significant that not all of the submissions would be processed before parliament breaks for the election. Those submissions would be handed back to their authors.

Guardian Australia has been contacted by one man who is frustrated that his submission supporting Labor’s policy, received by the secretariat on 3 March, is yet to be uploaded to the inquiry homepage.

Wilson said he was pleased by the significant interest in the issue.

“The participation in this inquiry has been extraordinary – thousands attending hearings and making submissions – so much so the secretariat is struggling to publish them all,” he told Guardian Australia. “Too many Parliamentary committees have low participation, and I am elated we have been able to provide a pathway for participation into Australia’s democracy.”

The committee’s deputy chair, Labor MP Matt Thistlethwaite, said the situation was untenable.

“It appears that 20% of submissions to the inquiry were written by Tim Wilson, so any claims to impartiality or objective scrutiny is completely out of the window,” he said.

Thistlethwaite was also frustrated by the inquiry’s inability to consider all submissions, saying it was “completely unacceptable”.

“We get the draft report at close of business Friday and we’re supposed to make a decision on Monday without access to all of the submissions – that’s farcical,” he said.

“It underscores that this was never about an objective analysis of Labor policy, this was a political campaign.”

But Wilson said Labor simply wanted to silence opposition to its franking credits policy. He said the inquiry had ensured that voices critical of Labor’s policy had been heard prior to the election.

When previously asked how he would consider submissions containing text he had written, Wilson said individuals had clearly agreed to the content.

“If they send in a pro forma submission it is a statement they agree with and represents their view. As chair I must respect what people send in, not dismiss it.”