No details were released on how that was to be achieved, any cut in numbers or from which visa categories.
Net migration in the year to September was about 70,000 but has slowed in the past few months.
Ardern had said that the three parties, the other being the Greens, shared a common view that increased immigration was putting pressure on infrastructure.
Massey University sociologist Paul Spoonley, an immigration expert, said the policy changes could tarnish New Zealand's reputation overseas.
"The Government are going to have to manage how the international community view the downsizing of immigrant numbers," Spoonley said.
"When we last decided there were too many immigrants - in 1996 - it took a couple of years to realise that the reputational damage and the reduced numbers both had negative effects on the economy. And we had changed our policy settings by 2000."
Spoonley said the international education sector may be hard hit by the changes.
"International education is a particular focus for the new Government, both in terms of moving from the volume emphasised by the last Government and moving to focus on high value courses and students," Spoonley said.
"Their own estimate suggests that this will mean a decline of $250 million out of the sector. But what will it do to the student accommodation market in the Auckland CBD?"
Spoonley said tougher transitioning from education to work and permanent residence might also make New Zealand a less attractive option for international students.
Given that approvals for those coming under the Skilled Migrant Category are the largest group of approvals, he said, a reduction by a third would also severely impact some employers.
June Ranson, chair of the New Zealand Association of Migration and Investment, said it would be difficult to comment until there was a clear understanding of where the reduced numbers of migrations were coming from.
However, she believed the new Labour-led coalition did not have a mandate for any immigration crackdown.
Ranson said the Greens electioneered on more of an open-door policy towards immigration and they did not agree to reducing migrant numbers.
"Does this mean the Greens have changed their mind as, based on the facts we have, there is no mandate in place," she said.
Eric Chuah, founder of Cultural Connections and former head of migrant banking at ANZ, said cutting 30,000 net migration was a big call.
"It's cutting the current net migration by 41 per cent," he said.
"The Government needs to think very clearly on the positive and negative impact of such drastic change."
