Ardern is attending the opening session of the East Asia Summit in the Cambodian capital Phnom Penh.
She said the repression by the military government, including the execution of opponents, was unacceptable.
"Those who are seeking to see an end to the military regime have been executed. That is devastating and a stain on our region."
She called the Myanmar conflict one of the most vexed in the region and said it was testing the ability of countries to reach consensus.
Ardern said she was feeling the absence of former Myanmar leader Aung San Suu Kyi with whom she had spent time at previous summits, but who is now jailed.
The prime minister was also pressing for greater consensus about the Ukraine war and the need for the international community to adhere to rules-based order.
Ardern said countries in the summit had differing relationships with Russia which was limiting the ability to reach a consensus.
Russia is present at the conference, represented by foreign minister Sergey Lavrov. He was noticeably well distanced from President Joe Biden at a pre-conference dinner.
Ardern was seated four places away from Biden at the dinner.
Neither of the heavyweights was at the opening session of the summit - they were represented by lower ranked officials.
Ardern has said she will not directly engage with any Russians at the summit, but if she should come across them will repeat New Zealand's unconditional opposition to and condemnation of the Ukraine invasion.
