Key Points:

If there is any referendum on child discipline laws it will be held by postal ballot in 2009, Prime Minister Helen Clark said today.

Early this week campaigners handed in their second attempt at a petition to force a referendum on the so-called anti-smacking law.

If the petition has enough valid signatures there have been calls for the referendum to coincide with the election, which must be held by November 15.

Miss Clark told Parliament the Justice Ministry had advised that should a referendum be held it should be taken as a postal ballot in 2009.

"The Justice Ministry has given strong advice based on the experience of conducting such referenda at the time of the 1999 general election," Miss Clark said.

They advised that it caused voter confusion, congestion in polling places and delayed the count.

To hold a referendum in conjunction with the election would involve more staff, polling places, training and organisation.

Miss Clark said a decision would had to have been made two months ago in April to hold the referendum alongside the election.

The relative cost of holding the referendum next year would be "roughly the same" if it was held along with the election.

National Leader John Key said if electoral officials could handle the complexities of a snap election they should be able to cope with a simple referendum question in conjunction with the election.

He accused the Government of using "alleged technical difficulties to suppress the democratic will of the New Zealand people", while Miss Clark said Mr Key's attack over the referendum was designed to "disguise" the fact he voted for the legislation in question.

Green MP Sue Bradford suggested that the review of the law next year, as required by statute, would fit well with a referendum, but Miss Clark said this had no relevance to the timing of the referendum.

Petition organisers say they have 390,000 signatures backing the call for a referendum on the question: "Should a smack as part of good parental correction be a criminal offence in New Zealand."

The Office of the Clerk in Parliament has two months to check the petition is signed by 10 per cent of registered voters or around 290,000 valid signatures.

If the threshold is met, the Government would have one month to name a date for a referendum.