Chinese media has issued a shot across the bows, warning that if Prime Minister John Key raises the issue of the South China Sea, he could risk compromising New Zealand's relationship with China.

But talking to media ahead of high level talks with China's premier and President over the next two days, Key said New Zealand's position would remain unchanged.

Prime Minister John Key touched down in Beijing around 2.30am (NZ time), accompanied by his wife Bronagh and son Max, to lead a trade delegation representing some of New Zealand's largest primary produce exporters, for talks focused on upgrading an eight-year-old free trade agreement (FTA).

REUTERS A Chinese Coast Guard ship (top) is seen near a Vietnam Marine Guard ship in the South China Sea, about 210 km (130 miles) off shore of Vietnam, in this May 14, 2014 file photo. From listening posts to jet fighter deployments and now surface-to-air missiles, China's expanding facilities in the Paracel Islands are a signal of long-term plans to strengthen its military reach across the disputed South China Sea.

While in China, Key is set to meet with Premier Li Keqiang tonight (local time) and tomorrow, will hold talks with President Xi Jinping.

But before he touched down, his visit prompted a warning from China's state news agency, not to raise issues of conflict in the South China Sea.

Key said he would not back down from raising the issue.

Lawrence Smith New Zealand Prime Minister John Key has begun a week-long trade trip to China. Chinese media have issued a warning for him not to mention the South China Sea dispute, which he is likely to do when he meets President Xi Jinping tomorrow night.

"I take it all with a grain of salt, there are many opinions out there. But New Zealand's position on the South China Sea hasn't changed, it's been consistent and we consistently raise that message with Chinese leadership," he said.

In what appeared to be a coordinated publication of the warning, both the Xinhua publication and the Global Times warned any mention of the issue could risk complicating the trade relationship between the two countries.

"Key should be reminded that New Zealand is an absolute outsider in the dispute and not a concerned party, and that any attempt by Wellington to break its promise not to take sides on the issue would risk complicating the flourishing trade ties between China and New Zealand," a Xinhua editorial said.

It also raised questions over why New Zealand is taking part in a military exercise, hosted by one of China's opponents Malaysia, in the region, at the time of his visit.

Key said it was a standard operation.

"It's part of a standard routine operation, that they fly there from one country to another fundamentally - we're not actually really, really close to the disputed territory.

"But they are over the South China Seas - we've done that before from time to time, depending on the availability of the Oriens, but it's not strategic that it's specifically happening now, it's just part of a routine exercise."

China's English-speaking Global Times newspaper ran the warning on the front page, saying a "signal" had been sent to the Oceania Prime Ministers. Australian Prime Minister Malcolm Turnbull lead 1000-strong delegation last week.

The Global Times story suggested both Prime Ministers were invited at the same time, so China could reinforce a message it did not want external countries involved in the dispute.

"The US has been trying to convince Oceania countries such as Australia to join its patrols in the South China Sea.

"China aims to use the visit to gain Australia's and New Zealands understandings on the regional situation, as well as to warn the US against ramping up sea tensions."

Key was unconcerned a speech by Foreign Affairs Minister Murray McCully earlier this year, appeared to go further than New Zealand's previous stance and criticise China's position.

"I think we take that all with a grain of salt, I think the main point that the Foreign Minister was really making is that there is always a risk of accidental problems, if you like, in the South China Seas, where young junior sorts of people misinterpret what's going on and then all of a sudden you have a problem."

New Zealand's preferance was if the parties could resolve the issue themselves, perhaps through arbitration.

Key and his family were welcomed into Beijing overnight.