Prime Minister John Key needs to distance himself from people who are "donkey deep" in work involving foreign trusts, Labour leader Andrew Little says.
Mr Key yesterday made an embarrassing disclosure of a link to a company specialising in foreign trusts, a day after kicking off an investigation into whether such trusts were too secretive.
The latest register of MPs' interests, published yesterday, showed Mr Key had a short-term deposit with Auckland-based trustee company Antipodes Trust Group.
The company described itself as a specialist provider of services for foreign trusts based in New Zealand.
This morning, Mr Little said the disclosure needed to be followed up by action. Mr Key was "clearly associated with people who are donkey deep in them [foreign trusts]," he told Radio New Zealand.
"He has got to distance himself from those people, and he has got to be very clear that this type of device - which is about allowing the mega wealthy of the world to avoid their tax obligations in their own countries - is not something that New Zealand is going to be part of. It is just wrong."
On its website home page, Antipodes Trust Group says New Zealand offers a "tax neutral environment for New Zealand-based foreign trusts which are a well-established vehicle for carefully managing the inter-generational transfer of wealth".
It points to tax-free benefits, laws that protect client confidentiality and "limited" reporting requirements which mean the identities of settler and beneficiaries do not need to be disclosed - the exact issues which the Government's review is looking at.
Antipodes Trust Group's executive director, Ken Whitney, is Mr Key's lawyer.
A spokeswoman for the Prime Minister said his short-term deposit was used to pay for any costs incurred from his trusts.
Mr Whitney had changed firms in the past year and the short-term deposit had moved with him, which is why it showed up in the register.
Mr Key's link to the firm does not indicate any wrongdoing. But the timing is unfortunate for the Prime Minister, coming as New Zealand's foreign trusts and transparency are in the spotlight as a result of the Panama Papers document leak.
On Monday, Finance Minister Bill English launched an expert review of the foreign trust regime, led by tax expert John Shewan.
The review was immediately dismissed as a "whitewash" yesterday by Opposition MPs because of its narrow focus and Mr Shewan's past work on defending businesses in tax cases.
Green Party co-leader James Shaw said Mr Key's latest disclosure reinforced the need for a full, transparent inquiry.
Mr Little challenged Mr Key to follow British Prime Minister David Cameron's lead and release all of his personal tax records. Mr Key resisted, saying that to do so would lead to increasing creep into MPs' personal affairs., including their spouses. He expressed "total confidence" in his investments, most of which are held in a blind trust.
Mr Little tabled his own records dating from 2010, and said the Prime Minister also needed to come clean.
"He was the one who said [about spying legislation] 'Nothing to fear, nothing to hide'. I think the same applies, possibly even more so about tax affairs."
Mr Key shot back in the House, saying the Labour leader should be tabling his CV instead "because he will be out looking for another job soon" - a reference to Mr Little and Labour's recent poor poll results.