Labour's Trevor Mallard has been left with a slightly red face after a blogger obtained an email Mr Mallard sent to party campaigners on how to sell the party's new tax policy.

In the email posted on Cam Slater's Whaleoil blog, Mr Mallard said the key thing was not to get "dragged down into the detail on the [capital gains tax]. The public don't care and we get boring."

It was used as ammunition by National yesterday, with associate finance minister Steven Joyce issuing a statement claiming Labour's new policy would add $18.5 billion to debt.

Mr Joyce added it was no wonder Mr Mallard was urging his colleagues not to start explaining the detail: "The fact is they don't stand up to any scrutiny."

Labour tightened its internet security recently after Mr Slater obtained information in databases on the party website which were not well protected.

Mr Mallard said yesterday he was not excessively concerned about how Mr Slater - a right-wing blogger - got the email. He said it was sent from a different computer system, was distributed widely and he expected Mr Slater was simply forwarded it at some point.

"The fact people send stuff on to friends who send stuff on to him is not anything that causes great concern."

He would not say who it was sent to originally but said Members of Parliament were sent a different version.

The email was sent last Friday and also warned Labour campaigners they would face "hiccups" including a TVNZ poll released last night. He said that poll was taken before the details of the policy were released, adding "we want to keep the momentum going as much as is possible".

Mr Mallard said to use social media to spread the word and attached an "avatar" - a picture to put on their Facebook and Twitter profiles - to use with "Own our Future" on it.

Mr Mallard also suggested "lines" and told them to re-tweet supportive comments from commentators.

There was a series of tweets on Saturday by Labour MPs claiming to have met people enraptured with the capital gains tax.

Kris Faafoi said "plenty of positive interest in CGT at the bar at Norths Rugby Club!" and Moana Mackey reported: "I can confidently say that thanks to Labour's tax plan if an election were held tonight in Countdown Gisborne we would be Government."

Others included conversations with a taxi driver to Jacinda Ardern meeting a man with five houses "who gave me a long list of reasons why he supports a CGT".

The tweets were fairly silent on the matter yesterday - after the blog went up - but Mr Mallard denied he had sent out a message to halt them.