The Finance Minister is defending the CTU's controversial attack ad, claiming it's a technique favoured by both sides.

Billboards and newspaper ads from the Council of Trade Unions describe Chris Luxon as 'out of touch' and 'too much risk'.

Finance Minister Grant Robertson says it's the CTU's right to highlight relevant issues in a democratic society.

"And they wouldn't be alone in doing that, the Taxpayers' Union have done numerous screengrabs of me, unflattering photos- and they're a surrogate of National and the ACT Party."

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