"Before I got back to the beach I saw people further up pointing. I thought they were warning me about a rip, but there was another boy, about 100m out from the surf," he said.
He went back into the water, this time accompanied by a surfer with a board.
"He was on his back and kicking too, but he was going under," he said.
"He disappeared 10 or 15 seconds before I got to him but I managed to grab hold of hair, then his hand, and pulled him back to the surface. He was crying by the time we got him on the surfboard, which I thought was a good sign."
Mr Andrews, who was still emotional and near exhaustion an hour after the rescue, said his 17 years in the military had stood him in good stead.
"I told myself to keep calm and stay focused," he said.
"I wouldn't have been any use to anyone if I hadn't kept it together."
Police said the boys were in imminent danger of drowning when Mr Dudley rescued them.
They were checked by St John medics but did not require hospital treatment.
Others have not been so lucky on Ninety Mile Beach this summer.
Sixteen-year-old Jarod Lineses of Auckland died after he was caught in a rip at Hukatere and swept out to sea on December 27.
A 61-year-old who collapsed while surfcasting is thought to have died of medical causes.