Nearly 80 per cent of donations to the National party over a three-year period were from undisclosed sources, research by the Parliamentary Library shows.
The research, commissioned by the Green Party, showed that parties pocketed $6.5 million between 2011 and 2013 from donors whose identities were kept secret.
This amounted to 58 per cent of all party donations.
Electoral laws say that a party does not have to publicly disclose the identity of a donor if their donation was less than $15,000 in a year.
But Greens co-leader Russel Norman said the high proportion of undisclosed donations was bad for democracy.
"Since National has come into power there have been several scandals where questions have been raised about the link between political donations and political favour.
"National Party Cabinet Club cash for access, Maurice Williamson and Donghua Liu, Judith Collins and Oravida, John Key and SkyCity are all examples where money and politics have intersected."
Dr Norman said National was the "worst offender" in collecting undisclosed donations.
Out of $4.4 million in donations, $3.4 million of them were from individuals or companies who gave less than $15,000 and fell below the threshold for disclosure.
"Almost 80 per cent of National's donations were undisclosed. They know who's paying them but the public doesn't," Dr Norman said.
The research showed nearly 70 per cent of the Labour party's donations were undisclosed.
Greens received $360,000 in undisclosed donations, around 32 per cent of its total donations.
Dr Norman said political donations needed to be more transparent and Greens wanted to lower the threshold for disclosure to $1000.
Figures were not yet available for donations made in 2014.
The Greens co-leader will discuss the research in the Rod Donald Memorial lecture this evening in Christchurch, which is titled "Democracy in New Zealand: who broke it, who bought it, and how do we fix it?"