The party declared income of $1.045 million for the 2011-12 financial year in its official disclosure to Elections ACT which was submitted last week.

But an accounting error meant that a donation of $400,000 from the party's investment vehicle, the 1973 Foundation, was not declared.

The foundation, a property investment trust set up to end the party's reliance on poker machine money, declared the donation on its return to the electoral commission under the rules governing ''associated entities''.

The party's ACT branch secretary Elias Hallaj told The Canberra Times that the error was reported to Electoral Commissioner Phil Green as soon as it was discovered.

''I immediately contacted Elections ACT to let them know that we'd noticed an error and we were going to amend it as soon as possible,'' Mr Hallaj said.

''We left off at least two line items.

''It's pretty embarrassing, but there was no malice and we're not trying to hide anything, it was just a mistake.''

The party secretary said that an amended return, audited by an accountant, would be lodged early this week.

Mr Green said that he had accepted Labor's explanation for the omission and said that mistakes on party returns were commonplace and parties were allowed to make amendments after disclosures had been lodged.

''We do audit these returns, usually about a month after they are lodged, and that's the most likely time when we find what are usually fairly technical mistakes, and then we will accept amended returns.

''But sometimes they [the parties] will come to us of their own volition and let us know if there's a problem.''

But the Canberra Liberals were less forgiving, with Treasury spokesman Brendan Smyth accusing Labor of having trouble adding up.

''I'm not surprised ACT Labor's embarrassed, given they take thousands of dollars from problem gamblers and aren't even honest about it,'' Mr Smyth said.

''This government has serious problems with numbers.''