Hall (20) won the title with a final throw of 45.47m to beat Olympian Shaka Sola, of Samoa.
It was a remarkable performance by Hall because he learned of his selection just two weeks before the championships.
"We were in the middle of a build-up and were just concentrating on strength training and drills," his coach, Raylene Bates, told the Otago Daily Times.
"He hadn't thrown the discus since last season."
Hall began a crash throwing programme to get ready for the championship and burnt his index (throwing) finger when practising with a frozen discus.
"Marshall then ripped the top off his throwing finger when throwing last weekend and had to use his middle finger at the championships," Bates said,The wound was deep and would take time to heal but Bates was confident it would be fine for the athletics season, which begins in October.
Hall took the lead with his first throw of 44.79m but Sola looked to have won gold when he threw 45.14m with his final throw.
Hall responded with his winning throw of 45.47m.
His sequence of throws ran: 44.79m, 43.33m, 41.41m, 43.91m, foul, 45.47m.
Hall won the national senior title in Wellington in March with 49.59m.
"I'm rapt by Marshall's performance," Bates said.
"We had not planned for Marshall to go because it was outside our budget. But we changed our mind when Athletics New Zealand paid half the cost."
Hall, the top-ranked discus thrower in the country, improved by 3m when he won in March.
