The addresses were collected from 20,000 postcards the Ne' />

Labour has been caught harvesting email addresses from a union's protest to Parliament.

The addresses were collected from 20,000 postcards the New Zealand Educational Institute teachers' union sent to Prime Minister John Key over early childhood education concerns.

Key refused to take the postcards so Labour MP Sue Moroney took them.

Now blogger Cameron "Whale Oil" Slater has found about 6000 email addresses in an online database used to send Labour Party political messages. The database was among a vast amount of sensitive data Slater found unprotected on Labour Party websites.

NZEI national secretary Paul Goulter told members the postcards had been given in "good faith" and Labour was not allowed to use the information for other purposes. He said Labour was told the union was "very concerned" and demanded the emails be deleted.

Moroney said the email addresses were used to tell people more about early childhood education but Slater said the database appeared to show the addresses had been used for other campaigns.