He posted about the "special night" on social media with a photo of himself, looking gaunt, alongside Ardern.
"Why are you so skinny dude?" one person asked on Instagram.
"Yayur!!! Looking slim Stan ;) watch out though the aunties will cook boil ups and tell you to go eat," another said.
Ardern also discussed Walker's health on Instagram, thanking him for attending the Youth Programme event "despite being unwell".
Fans have been asking on Facebook after his recent dramatic weight loss for weeks.
A person claiming to be a distant cousin explained the situation, saying: "His weight loss is due to heath reasons - not by choice, coming from a distant relative who also has the same health issue. Upsetting seeing lots of posts with negative comments from people when they don't even know the reason for the weight loss."
Walker postponed a tour of New Zealand in 2016 as his mum April Walker recovered from breast cancer.
He had previously discovered he inherited a rare cancer-causing gene mutation called CDH1 that has caused the deaths of more than 25 of his family members, according to Fairfax.
In October last year Walker told the Herald his mum was an inspiration.
"Mum said, 'I never thought cancer would get me,' but it did. She's an inspiration, and now that she's in recovery, she's focusing on speaking about her health journey, particularly to Maori-Pacific women, and raising awareness of cancer."
