NAB directors quietly agreed to stop donations at a board meeting in May, stemming a tide that has seen more than $500,000 flow into political coffers over the past three years.

The Big Four are among the country's biggest and most reliable donors, contributing more than $5 million to the Coalition and Labor in the past decade, according to parliamentary library research.

But the sun is setting on this pot of gold. Appearing before a parliamentary inquiry into the banking sector on Wednesday, ANZ boss Shayne Elliott indicated his bank was also reviewing its donations policy.

"We are debating, we are having discussions at our board about the role of political donations and what our position is on that," he said.

ANZ has donated $1.65 million since 2004-05, usually split evenly between Labor and the Coalition, although it gave more generously to the Coalition in 2013-14 and 2014-15.

NAB has donated more than $1.8 million in 11 years, including $239,686 to the Coalition and $35,600 to Labor last financial year.

This year the bank sponsored a major post-budget fundraiser for cabinet minister Kelly O'Dwyer, where guests paid upwards of $150 for breakfast at the Crown Casino's Palladium Room.

Fundraising activities are now banned under the bank's new directive, along with all monetary and in-kind gifts, and the payment of entry fees for party-political events.

"NAB does not make donations or contribute funds to any political party, parliamentarian, elected official or candidate for political office," the policy document declares.

In a further move, any employee wishing to attend a political meeting or activity as a NAB representative must clear their invitation with the bank's Government Affairs and Public Policy division. Any exceptions to the rules need to be approved by the board.

Westpac said that since 2008 it did not make cash donations to political parties, but it paid for involvement in political activities that had "direct business relevance to Westpac". That expenditure amounted to $176,455 in 2014-15, records show.

The Commonwealth Bank also precludes itself from cash donations but gave almost $100,000 to the major parties in 2014-15 for attendance at political events. Neither bank said it was reconsidering its policy.

Graeme Orr, political law expert the University of Queensland, said the "slow build-up of stench around political money" meant the writing was on the wall for large corporate donors.

"There's been a sense that while money may buy some us influence and ingratiate us, but it also may make us look tainted," he said. "This is a slow trend in Australia and I think it will continue."

Jo o-Cheong Tham, director of the Electoral Regulation Research Network, said many corporations were "pulling back from that arena", particularly because of the donations scandals in NSW.

But he warned: "Big companies like the four banks don't need to pay money to access senior ministers. They will be heard."

And constitutional law expert George Williams argued any decision to stop donating was most likely made for commercial rather than altruistic reasons.

"I don't see a general trend here. People donate money because they get a return on it." he said. "The desire of the parties for large sums of money continues unabated."

Asked whether Labor should continue to accept money from the big banks, Opposition Leader Bill Shorten said it was a matter for the party's administrators.

Under questioning from Greens MP Adam Bandt, Mr Elliott told the parliamentary inquiry that ANZ justified its donations "on the basis that we are supporting the democratic process".

Mr Bandt suggested ANZ should consider those donations "a pretty good investment" because of what he portrayed as advantageous political treatment over the past decade, including avoiding a royal commission. Mr Elliott said the bank did not view donations as "investments".

Experts also warned that waning corporate cash would not necessarily be replaced by a cleaner system, with parties likely to deepen their dependence on wealthy individuals.

"The risk for corruption is actually increased with the remaining contributors. You're heightening the reliance on a smaller number of donors," associate professor Tham said.