His office said two members of the public helped him and he continued to parliament, attending a meeting before going to hospital for a precautionary check-up.
He suffered a black eye, telling stuff.co.nz "it looks worse than it is".
Police said a 47-year-old man was arrested and would appear in court Friday charged with injuring with intent.
Political violence is practically unheard of in New Zealand, a nation of about 4.5 million where most government ministers do not have a regular security detail.
"We have an environment in New Zealand where politicians are accessible, and that's something we should feel proud of," she told reporters.
"This is outrageous behaviour and if undertaken by a Wellingtonian, shameful," he tweeted.
"Elected representatives of all hues show courage to represent New Zealanders and we are lucky to have unparalleled access to them."
There was condemnation from across the political spectrum, with the leader of the conservative National Party Simon Bridges tweeting: "Violence is never OK."
Ardern said Shaw had told her he was doing fine" and she had told him to take whatever time off he needed to recover.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)