On Tuesday, Fonterra announced it would build a new ultra-high temperature processing (UHT) cream plant at its Edendale site, which is expected to create 70 jobs.
Mr Duffy said the investment will be great for Edendale and the wider Southland community.
"I think it’s a positive, every way we look at it."
The 70 proposed jobs would add to the current 670 employees.
"That’s very good, not just for the local community but wider Southland."
Additional employment opportunities will also be created through the construction phase of the plant, which is scheduled to start early next year.
The investment signalled an ongoing monetary advancement of the plant and diversification in its products, he said.
His reaction to the news had been tame, as he had a "bit of a heads up" and had been told earlier that the investment was "likely".
"We just overlook [the factory] quite often and forget that it is the largest dairy factory in the country."
It was the largest fresh milk processing plant in the world.
In the 1960s, he worked at the Seaward Downs Dairy Factory before it merged with the Edendale plant.
Fonterra lower South Island regional manager Andrew Johns said the investment future proofed the site.
"It’s fantastic for the community.
"The continued investment means a lot for us."
Ideally, many of the new jobs would go to local people.
"We’ll work with local businesses and Southland District Council, Great South and other avenues to acquire the best people."
He hoped to attract people to Southland internally from the business.
The project had been in the works for "some months" and a lot of hard "mahi" had gone into it.
"It’s been pretty special for everyone these last 24 hours.
"It feels really good.
"There’s a lot of hard work to go."
News within the community travelled fast, as 30% of the workers lived in Edendale, he said.
"The general reaction is very positive.
"So I’m very comfortable saying that it’s a good outcome.
"The future of the Edendale plant is certainly secure."
The new plant will initially create upwards of 50 million litres of UHT processing capacity with the capacity to grow beyond 100 million litres by 2030.
Up to 15 million litres of milk per day was already processed at the plant.
The newly created whipping cream will be used in about 260 million cakes in Chinese bakery stores each year as well as 400 million beverages.
Southland Mayor Rob Scott said it was "fantastic" news.
"The addition of 70 jobs in that plant is awesome."
The first product is expected to come off the line in August next year.
