And it was a time that helped shape his career, given that Marist coach Doc Cournane switched him from a lock to his current position of prop.
"I was a lock, because I wasn't really heavy enough at that stage," said Hayes.
"But then, over the first year when I was in Invercargill, I started putting on a bit of weight and Doc always used to be on to me, saying 'you should be a prop.'
"After a while, he started getting serious about it and started pulling me in to scrummage against him. He showed me what to do.
"I played the first season all at lock, and then the second season I started off at lock again, and it was the last four or five matches of that season when I changed over."
Hayes, now firmly established in the Ireland side, has fond memories of cutting his rugby teeth at New Zealand's southern-most outpost.
"I enjoyed every single minute of it," the 31-year-old added. "I've never enjoyed any two seasons so much, and I couldn't learn enough. It was just brilliant.
"I played Southland Colts rugby the first year I was here. I was only 21, and that was great, so it's great to be back down here again and getting to play against Southland."
As for the local oyster delicacies though, Hayes will be giving them a wide berth.
"I never liked them," he said. "Even when I came out first, they were all giving them to me and I didn't really like them. I just don't like seafood - all I want is beef and lamb."