In their talks, Ardern made a few non-controversial opening comments about having had diplomatic relations for 60 years and how she was looking forward to having discussions particularly on regional security issues.
Mahathir, who had a reputation for straight talking when he was last Prime Minister (1981 – 2003) had very little small-talk.
He said there were no problems between the two countries. He mentioned Malaysian students studying in New Zealand then launched straight into his comments about China.
"Although New Zealand is quite far from the South China Sea, what happens to the South China Sea will eventually have some impact on other countries around there. Of course we are very close for us.
"The freedom of navigation in the South China Sea and the Straits of Malacca is very important and we hope that by everyone showing interest in this need for freedom of navigation and overflight, China might realise that it is wrong for them to do anything that may obstruct travel.
"All we believe is that while we accept that warships can come there but they should not be stationed there …"
There were about 20 journalists in the room, loudly taking photos and recording the comments but Mahathir had barely mentioned China, when Peters appeared to be trying to get New Zealand officials to get the media to leave.
Congratulations on the birth of your sweet baby girl. You are indeed a symbol of a society where women shine.
Speaking to journalists after the meeting, Ardern said the free movement in the South China Sea affected New Zealand as much as the region because it had vessels that passed through it too. She did not take Mahathir's comments as a warning.
"We didn't anyone to point out of course to us the importance of the issue for the region. We are very, very aware of it which is why we have consistently raised the South China Sea as well."
New Zealand had never taken sides in the conflicting territorial claims in the South China Sea and it believed everyone should uphold international law.
The countries with overlapping claims to China and China itself this week reached a significant agreement in the long-running dispute – a single draft Code of Conduct negotiating text, which will form the basis of negotiations.
Ardern said she thought it had been time for the media to go after opening comments.
Peters left for Apec in Papua New Guinea straight after the bilateral meeting.
Ardern also held talks with Japan's Prime Minister, Shinzo Abe, who congratulated Ardern on the birth of her daughter in June.
"Most importantly congratulations on the birth of your sweet baby girl. You are indeed a symbol of a society where women shine."
He also said many Japanese had been excited to see the All Blacks play in Japan recently.
She said later Abe had invited her Japan during the Rugby World Cup in 2019.