The Prime Minister is hitting back at opposition party criticisms of investor state dispute clauses in free trade deals.

John Key says Labour has to ask itself, if it's so opposed to investor state clauses in FTA's, then why did it sign the free trade deal with China?

He's also criticising New Zealand First and Winston Peters for being inconsistent by wanting to do away with investor state clause - but at the same time say they support the South Korea free trade deal.

"Either he doesn't believe in the bill that's in the name of his own party, in which case he should tell his MP to stop it, or he should be honest with the voters of Northland and say 'I'm telling you one thing before the by-election and I'm going to tell you something completely different afterwards'."