Labour is defending using a Facebook "baby number" widget to gather more than 54,000 email addresses, saying it is a way of starting conversations with people.
The widget offers people the chance to find out what number baby they are - calculated since the public health system was founded with the passing of the Social Security Act in 1938.
It assigns every New Zealander born since then with a birth number and in exchange, people give the party their email address.
Labour's move followed the National Party last month inviting people via Facebook and Twitter to wish Prime Minister John Key a happy birthday if they supplied an email address.
The 6000 people who did so were added to the party's mailing list.
Labour Party general secretary Andrew Kirton said it was a new way of communicating with people.
"We're a political party and we want to start conversations with people.
"It's completely optional, you can unsubscribe with two clicks - one click if you're on Gmail - at any time."
The widget is a copy of one created by the British Labour Party.
Privacy Commissioner John Edwards said such schemes were not illegal because people were told what their email address may be used for.
"There is a pretty clear statement telling people that they may be contacted by the Labour Party."
Mr Edwards said people were not being forced or tricked into handing over their email addresses "but it is a condition of engaging with this appealing widget".
Such gimmicks were increasingly common among political parties, he said.
The Green Party is currently circulating a petition on Facebook calling for greater funding for the Department of Conversation.
Those signing the petition are given the option of not receiving emails.