A former treasurer of the Liberal Party is calling for political donations to be capped at $500 to curb what he describes as "soft corruption".

Michael Yabsley, who founded the party's fundraising organisation, The Millennium Forum, said the current system is open to abuse.

"I've had conversations with many donors who have said, 'I want to give a substantial amount of money, but I would also like to see a particular minister or a particular political leader'," Mr Yabsley said.

"That is not a corrupt action in itself, but it's not a good look and I would call it soft corruption in a very, very small number of cases."

Mr Yabsley made his remarks ahead of the release of a report showing there is no way to identify the individuals and organisations behind more than $60 million worth of donations to Australian political parties in the 2016-17 financial year.

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Researchers from The Grattan Institute spent six months following the money trail and have released their preliminary findings to the ABC.

The director of its budget policy and institutional reform program, Danielle Wood, said the report highlights the lack of transparency surrounding political donations.

"We know there were $62 million dollars in income to parties last year and we are not able to tell where it's from," Ms Wood said.

"This view that big interests have special relationships and special access is bred by the secrecy around these numbers."

The report, expected this month, has also found there is no way of tracing the sources of 40 per cent of donations to political parties over the past decade.

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Anonymity the issue: Wood

Ms Wood blamed rules that currently allow donors of less than $13,500 to remain anonymous.

"The idea is you don't want to tie parties up with red tape having to disclose small donors and people have a right to privacy," Ms Wood said.

"One of the issues with that, though, is parties are not required to aggregate donations, so someone could give a whole series of donations under the cap but, collectively, they account for large amounts."

In cases like these, Ms Wood said the identity of the donor should be revealed to provide clarity and help restore trust.

"I think putting those donors on the record helps reassure the general public that people aren't able to purchase policy outcomes," she said.

Ms Wood said Mr Yabsley's idea to cap political donations is a good one, but she thought it could raise some constitutional concerns.

"It may impinge on the freedom of political communication," she said.

"If you had low caps and that substantially reduced the capacity of parties to raise money and therefore sell their political message, it may be seen as too restrictive."

Ms Wood thinks placing limits on the expenditure of political parties would have the same effect while being easier to monitor and enforce.

"What we want to do is reduce the reliance of the major parties on those big donors and cap expenditure during political campaigns," she said.

The Grattan Institute's report also reveals in the two financial years covering the 2016 federal election more than half of all donations to the major political parties which are disclosed were made by people and organisations representing just 5 per cent of all donors.

For Labor, the major benefactors are unions, particularly the CFMEU, United Voice, and the Shop, Distributive, and Allied Employees Association.

The Liberal Party received the bulk of its warchest from fundraising outfits like the Cormack Foundation and individual donors, including mining figure Paul Marks.