Relaying his story to the Otago Daily Times yesterday, Kalya said all had been well during a family outing to the beach about 2km from their home, when an unexpectedly large wave knocked his brother over while they paddled at the shore.
"A second wave came, then a third, and quickly Kithmi got pulled out.
"I knew he could swim, so I waited a bit for him to try and get back in by himself. But he kept getting pulled further out, so eventually I said to Mum, 'Okay Mum, I'm going out. I might not be back."
By this stage, his brother was about 60m offshore.
"The water was freezing and very rough, so I was a bit worried we'd both get stuck out there.
"Luckily, I remembered my life saving practice from [Sri Lanka], and we both made it back."
Kalya said he learned to swim in 25C waters in the Indian Ocean.
Typical winter temperatures off the Otago coast are about 10C.
Kalya's father, Daminda, said he had only been reunited with his family — the two boys, a daughter and wife Asha — a month ago, after four years apart.
Covid-19 and immigration complications had delayed any earlier reunion.
Kandegoda Gamage has worked on the Chrystalls Beach dairy farm of Tony and Sue MacDonnell for the past year.
Gamage said her sons were brave, determined characters, and took good care of each other.
"So many people helped on Saturday, and we're just so grateful our son was saved.
"We're very proud of the boys."
MacDonnell said ocean conditions on Saturday were the fiercest he had seen during his 10 years on the farm.
"The waves were 3m high, vertical, and slamming straight down on the shore.
"It's a miracle they came out okay."
