John Key. Pic: NZPA

Wellington, April 12 NZPA - Prime Minister John Key's chance to meet American Vice-President Joe Biden at the White House tonight is a "pretty special opportunity" which he will use to push New Zealand's trade interests, Acting Prime Minister Bill English says.

Mr Key has headed to Washington ahead of schedule after unexpectedly gaining a meeting with Mr Biden before a nuclear security summit.

"That's been an opportunity that's come up just in the last few days and John Key is taking (it) because it's a pretty special opportunity to push the trade negotiations that have just begun with the US," Mr English said on Newstalk ZB this morning.

Talks started last month in Melbourne on a Trans-Pacific Partnership (TPP) which would expand the previously negotiated P4 trade agreement between New Zealand, Brunei, Chile and Singapore to include the US, Australia, Peru and Vietnam.

A group of 30 US senators responded by sending a letter to US Trade Representative Ron Kirk critical of Fonterra and accusing New Zealand's dairy industry of "anti-competitive practices".

Idaho senators Mike Crapo and Jim Risch led 28 others in urging "very careful attention to dairy trade concerns" and said New Zealand's dairy industry wielded extensive control over world prices.

Mr English said Mr Key would use his meetings to outline New Zealand's position.

"The practical negotiations have just got started and of course negotiating any trade deal with the US is going to be a complex job requiring persistence so the Prime Minister's taking the opportunity to push our case every chance he can."

Mr English did not think the senators' letter was "that relevant" to trade talks.

"In our view the US government has a realistic view of Fonterra as an organisation that has to compete successfully in global dairy markets and the fact that it is done reasonably well just means that it's a good New Zealand organisation, not that it has any particular protection from the Government."

Trade Minister Tim Groser previously said the senators were influential but it was "palpable nonsense" to say Fonterra created an unfair market because it competed internationally like every other company.

"It's a very, very politicised argument, trying to suggest that somehow New Zealand doesn't play it fair when any person who looked at it objectively would reach exactly the opposite conclusion," he said.

Mr Key will also have a chance to speak privately to President Barack Obama during the two-day summit, which begins tomorrow morning (NZ time). After his talks with Mr Biden, Mr Key will meet Agriculture Secretary Tom Vilsack,