New Zealand has joined 13 other countries in supporting a 2016 court decision about the South China Sea — a patch of ocean near China that several nations claim parts of. China says this makes New Zealand less reliable as a partner and damages their relationship.

The court (called an arbitration tribunal) had ruled that China's claims to most of the South China Sea are not legal under international maritime law. That law, called UNCLOS, sets out the rights countries have at sea. New Zealand, Australia, Canada, the United States, Japan and several European countries all signed a statement agreeing with that court decision on its tenth anniversary.

China's ambassador to New Zealand, Wang Xiaolong, responded by saying New Zealand had gone against "an understanding reached by our leaders." He said the country needs to "cease immediately violating China's interests." China's government also released its own statement, putting the word "so-called" before the arbitration award — a way of rejecting it without accepting what the court decided RNZ.

Foreign Affairs Minister Winston Peters said New Zealand stood by the statement and had nothing more to say about it RNZ. That short response is how Peters has dealt with disagreements with China — he states the position and then does not argue about it.

The 2016 court decision said China's "nine-dash line" — the way China marks out the territory it claims — has no basis in law. For a decade since, countries sympathetic to that ruling have said so publicly. China has never accepted any of it.

This puts New Zealand in a difficult spot. The country believes strongly in international law and rules that apply fairly to all nations — especially important for a small country like New Zealand. But China is also New Zealand's biggest customer for exports. By signing the statement with allies like the United States and Australia, New Zealand chose to stand with its traditional partners rather than try to stay neutral.

China's ambassador said that New Zealand leaders had earlier promised not to sign statements like this one. It is not clear if this was a firm agreement or just something China raised that New Zealand never formally confirmed. Either way, China sees this as breaking a promise made at the top level, not just disagreeing about what the law means.

It is too early to say what China might do in response. In the past, China has sometimes punished countries that criticise it by making trade harder for certain products. For now, New Zealand's government appears to be closing the conversation rather than prolonging the argument.