The Prime Minister says the National Party agreed to a spending cap on how much third parties can spend on campaigns before elections in order to get bi-partisan support for new electoral finance law.
The Electoral Legislation Select Committee is proposing a spending cap of $300,000 on third-party election campaigns.
The committee has made the change in two bills reforming the electoral finance laws and setting up the MMP referendum.
The bills are the Electoral (Finance Reform and Advance Voting) Amendment Bill and the Electoral Referendum Bill.
John Key says there is always a bit of give and take when trying to get cross-party consensus on legislation.
He says the proposed spending cap will allow third parties to have their say without being overbearing on political parties.
An academic specialising in political finance believes lobby groups will still be able to get around proposed rules to restrict spending on third-party campaigns.
Otago University politics lecturer Bryce Edwards says spending caps are very easy to get around.
Having limits on campaign expenditure creates ongoing problems for political finance regulation, as loopholes will continue to appear, he says.