New Zealand's alternative national flag grabbed the spotlight at the historic signing of the Trans-Pacific Partnership in Auckland today.

Prime Minister John Key wore a silver fern pin on his lapel as the world watched New Zealand and 11 other countries sign off the trade agreement after five years of negotiations.

The black, white and blue flag was clearly displayed while Mr Key gave his speech to the audience at SkyCity casino and was broadcast to an international audience.

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Twelve Pacific rim countries have signed a sweeping trade deal in New Zealand today. Hailed as the "biggest trade deal in a generation", the TPP is designed to free up trade and investment between the countries.

The current national flag also made an appearance at the TPP signing, albeit in a less prominent position on the stage behind member countries' trade ministers.

Mr Key has worn a silver fern pin for several years, but rarely at such a high-level occasion.

He has begun wearing the fern emblem more regularly as the March referendum on a national flag approaches.

The Prime Minister has long campaigned for a change of national flag, and has made no secret of his preference for a silver fern design.

New Zealand First leader Winston Peters, who opposes a flag change, said Mr Key was free to wear whatever he wanted.

"However, I don't think portraying himself as a show pony for the new flag option is going to cut any ice with the New Zealand people," he said.

The choice of lapel pin also prompted discussion on social media, with several commenters questioning why the Prime Minister was not wearing New Zealand's official flag at such a major occasion.

Mr Key said last year that he wore the silver fern because "it, to me, symbolises this country that I love and so proudly serve".

He said many New Zealanders, such as All Blacks fans, had already adopted the fern.

In doing so, he said, "they have adopted a symbol that unites them as belonging to a young and proudly independent country that has achieved a lot and has more to do".