The national Milk in Schools programme has been launched in Southland and Otago, with at least 19,000 primary school children receiving a daily carton of milk.
The scheme began on Tuesday and is being run by dairy cooperative Fonterra following a pilot in Northland last year.
Kerry Hawkins, the principal of Waverly Park School in Invercargill, says many families in the city are unable to afford to give their children all the nutrition they need.
"We hear a lot about the achievement gap in schools - but the real problem in New Zealand is not an achievement gap, it's a poverty gap. And so this goes some way to bridging it.
"So the fact that this was Fonterra - no disrespect intended - is irrelevant. What is relevant is that here was a company willing to put their money where their mouth was."
Fonterra will roll the scheme out nationwide during the rest of the year, with about 350,000 children in 2000 schools expected to take up the offer.
The cost of the programme is estimated to be about $20 million.