He told RNZ residents were shocked and astonished that such a crime could happen in their town, and that was reflected in the protest.
They are calling for the provincial government to put a new security and crime prevention plan in place, Mr McGarvey said.
The provincial security minister had asked to meet the protest's organisers next week to discuss the issue.
According to El Sol Online, a 19-year-old was arrested in connection to the killing in the neighbourhood in upper Mendoza following police raids.
He was due to be released shortly, El Sol Online said.
After the offer of a 40,000 pesos ($5800) reward for information leading to the capture of the assailants, a witness came forward on Monday night.
Mr Heyward was born in Australia but his late mother was from New Zealand. He also travelled on a New Zealand passport.