The company says it will start distribution for Molson Coors in New Zealand "from mid-2012" with a plan to re-enter the Australian brewing industry two years later.
Alcohol Healthwatch director Rebecca Williams last night said: "We need more alcohol like we need a hole in the head."
"Any new brand will be accompanied by aggressive marketing strategies," Ms Williams said.
"Alcohol is causing a significant problem already and we're grappling with getting the best answers."
Coca-Cola Amatil told investors this week it would enter New Zealand's market with "international partners", the first being Molson Coors.
A Molson Coors spokesman in the UK said it was the first time the company's beers were to be sold in New Zealand.
Coca-Cola Amatil is the Australian-based bottler and distributor of Coke and other soft drinks.
The company's six-point plan to break back into the Australian brewing industry includes new strategic partnerships with Fiji-based beer brands.
The company believes there is more than A$1 billion ($1.29 billion) a year to be made in the Australia and Pacific region and a further A$200 million in the premium beer market.