Companies and shareholders associated with 12 fast-track projects gave more than $500,000 in political donations to National, Act and New Zealand First and their candidates, RNZ analysis shows.

The projects include a quarry extension into conservation land and a development whose owner was publicly supported by National MPs during a legal battle with Kāinga Ora.

University of Otago legal expert Professor Andrew Geddis said the Cabinet Manual doesn't cover whether receiving a donation is a conflict of interest.

In a political system where a lot of the money used for campaigning comes from private sources, it would be difficult to bar parties from being involved in decisions which impact donors.

"The unions give lots of money to the Labour Party. If that was then to mean that the Labour Party couldn't pass workplace relations legislation that impacts on unions, it would take a huge policy issue just out of play."

If this was the rule, donors wouldn't donate, he said.