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.