The Prime Minister is defending the appointment of Julie Christie to the Flag Consideration Panel, saying any conflict of interest has been managed.
In documents released under the Official Information Act, Ms Christie declared a conflict of interest regarding her board membership with New Zealand Trade and Enterprise.
She sits on a committee which promotes the use of the Fern Mark, the black and white silver fern, as a branding symbol for New Zealand.
Three of the four designs for an alternative flag featured the silver fern.
Speaking from Papua New Guinea, John Key said government ministers had to regularly manage potential conflicts of interests.
"It's how you manage the conflict of interest, she seemed to indicate she'd managed it."
He rejected the suggestion Ms Christie's membership of the panel skewed the result.
"It's hardly a shock that you're going to have a significant number of silver ferns, on the basis that if you go and have a look at the final 40, for a start-off, there was a range of either ferns and korus and then a broader sort of group.
"And anyway the fern's been a very strong argument, I think, around why people would want to change as an indentifying and kind of unifying symbol of New Zealand we've used for well over 140 years."
The documents also revealed the Flag Consideration Panel gave an advisory group, including two designers, just four hours to give their advice on the flag options.
Labour Party Arts, Culture and Heritage spokesperson Jacinda Ardern said the papers showed the Panel appointed the group just a month before the final four designs were released.
Ms Ardern said that seemed like a token gesture in response to public complaints about a lack of design input in the flag process.
She said the group, which included a Nike creative director, recommended that more geometric designs be included in the longlist.
A New Zealand Flag Consideration Project spokesperson confirmed the advisory group had a four-hour meeting with the panel and found its advice constructive and helpful.