Trade Minister David Parker says New Zealand won't be moving quickly to join any WTO dispute against the United States on its tariffs against new steel and aluminium imports because too much is still uncertain.

Even those countries which with "exemptions" from the 25 per cent tariff on steel and 10 per cent on aluminium, including Australia and the European Union, may be facing exemptions which are temporary until May 1 and conditional.

"We don't know what the outcome is in the position in respect of steel and aluminium for New Zealand or for other countries. It is still a moving feast," Parker told the Herald.

President Donald Trump last week signed off the tariffs ordered under the auspices of a "national security" justification, and the exemptions.

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New Zealand Prime Minister Jacinda Arden has written to Trump seeking an exemption for New Zealand's small but important $60 million export market of steel and aluminium to the US and Parker has written to his counterpart, Robert Lighthizer.

The US position has been complicated by the conclusion of a 301 inquiry under the US Trade Act inot trade practices by China, such as forcing US companies to share IP, under which retaliatory action to the tune of $60 billion has been foreshadowed against China by the US.

Part of that controversy is that the US is using an outdated law (used frequently by Lighthizer under the Reagan Administration) instead of using the disputes rules under the WTO (World Trade Organisation) which only came into force in the 1990s.

The WTO believes its support is needed before invoking 301 retaliation.

"We don't know where the 301 inquiry in respect of other issues which came out on Friday is going to end either," said Parker.

New Zealand's representative took part in a debate at the World Trade Organisation at the weekend expressing concern at the United States moves toward protectionism. It was one of 19 countries that spoke. Others included Japan, Brazil, Japan, China, Russia, South Korea, Singapore and India.

Even those countries which have exemptions from the 25 per cent tariff on steel and 10 per cent on aluminium, including Australia and the European Union, have been told those exemptions may be temporary until May 1 and may be conditional.

Parker said he had been broadly briefed but not on what New Zealand had said at the WTO debate

"We are being careful in saying that we believe in the importance of the WTO but we are taking a position as to what we might do in the future because it depends what happens.

"We reinforce our belief in the multilateral rules," he said. "We are obviously trying to defend New Zealand's interests in respect of the steel and aluminium issue, and in respect of broader issues as to whether there should be a WTO response we haven't yet take a position on that and probably won't for some time."

"We don't really know where this ends yet, even in the short term. It really has been a moving feast and it seems to change every few days."