The Prime Minister's long-time lawyer lobbied the Government to not change New Zealand's foreign trust rules last year, the Green Party says.
The Government put paid to the IRD reviewing the rules shortly after Ken Whitney wrote to the Revenue Minister claiming John Key had promised him the regime wouldn't change.
Correspondence obtained by the Greens showed Mr Whitney, a trust specialist, started lobbying on the issue.
"This shows how powerful vested interests can get the ear of the Government who will then change its plans to suit them," Greens co-leader James Shaw said.
"I want the Prime Minister to tell New Zealand whether he gave Mr Whitney an assurance that the foreign trusts industry would not be reformed and if so, why he hasn't been upfront about the assurances he gave."
He says normal members of the public don't get the same access or immediate response to their concerns.
Within 24 hours of Mr Whitney writing to then Revenue Minister Todd McClay, claiming the assurance from Mr Key, Mr McClay emailed the IRD saying he didn't want the foreign trusts industry to close.
The IRD review was shut down five months later, despite it having serious concerns to risks to New Zealand's reputation as a perceived tax haven, the Greens say.
The Government came under pressure about the country being a tax haven following the release of the Panama Papers which exposed the use of foreign trusts to avoid paying taxes.
It appointed former Pricewaterhouse Coopers boss John Shewan to investigate the country's foreign trust disclosure rules.
Mr Shaw says the inquiry needs to be widened to include a panel of experts and with the public to have its say.
A spokesperson for Mr Key says the allegations are "just more desperate claims from the Greens".
"People raise various issues with the Prime Minister all the time -- at events such as school and business visits, and informal gatherings all around the country, as well as in written correspondence.
"It is common practice for him to refer them to the appropriate minister, where appropriate, or to answer if he is able to do so."
New Zealand is already participating in an ongoing OECD work programme that's looking to address the issue, he says.