The Prime Minister is blasting a New Zealand journalist working on the Panama Papers as a "conspiracy theorist".
LISTEN: John Key speaks to Chris Lynch on Canterbury Mornings
The international scandal has seen New Zealand named as a favourite place for wealthy foreigners to hide money, using trusts.
Nicky Hager is among the journalists who received early information to investigate the issue.
John Key said Mr Hager is a conspiracy theorist, but won't elaborate on why.
"Because I think he is. You can draw your own conclusions."
Mr Key said allegations that New Zealand has turned into a tax haven are "utterly incorrect".
Andrew Dickens: Have no idea about the Panama Papers?
He said people might be hoping to find a tax haven here, but they'll be disappointed.
"One of the reasons we have that good reputation is because we're compliant, because we exchange information, because we honour requests when they're asked of us."
John Key said there's plenty of legitimate uses for foreign trusts, and they were already working on some of the legitimately concerning issues.
He said a Cabinet committee is currently investigating the rules for what's referred to as "look-through companies", as they had concerns about that area before the Panama Papers were released.
But United Future leader and former Revenue Minister Peter Dunne is warning the Panama Papers could damage our international reputation, and is urging the Government to act quickly.
He said National needs to tighten up the rules, even if it's only to tackle the perception problem.
"These things have an unfortunate habit of sticking. In this country we've always been very critical of countries we've perceived to be tax havens, and I think that the danger is we now might find, inadvertently perhaps, ourselves in that same space."
Mr Dunne said we should be reviewing our disclosure rules, and broaden our network of Tax Information Exchange Agreements.
He said it would be pretty straightforward and quick to put the amendments through.