Weak laws covering donations to political parties are undermining public confidence in the electoral system, says the Electoral Commission.

The commission's annual report says it is too easy to make anonymous donations and calls on the Government to change the law.

"The high disclosure threshold and the ability of donors to make large donations anonymously reduce the degree of transparency which the disclosure regime is intended to promote and thereby undermine public confidence in the integrity of the disclosure system," says the report.

The law allows one or more donations to a party in one year up to $10,000 before the donor must be revealed, but there are various ways to get donations declared anonymous.

The commission says a select committee inquiry into the disclosure of donations should start "without delay".

At the last election, Act spent $1.6 million on its campaign for a return of nine seats in Parliament, compared with Labour's $1.4 million for 52 MPs. Each seat cost Act $181,000.

Labour won 52 seats, costing the party $28,000 each. National spent $1 million for 27 seats at an average $49,000 each.

Political parties can spend $1 million each, plus $20,000 for each electorate they contest, making a maximum of $2.38 million if a party runs candidates in all 69 electorates.

Spending on items such as print advertising, billboards and pamphlets in the three months before the election is counted.

Television advertising is paid for separately by the taxpayer.

At the last election, National and Labour received $615,000 each, with Act, the Greens and NZ First getting $166,000 each.