More: BRUTAL knockout sees fighter fall out of the ring
Horn dominated the fight before an accidental head-clash in the sixth round caused the bloody cut.
The fight was delayed as the ringside doctor examined the cut before the start of the seventh round, eventually letting the bout continue.
But after the end of the seventh, the same doctor instructed referee Lance Revill to stop the fight as the cut was too bad.
There was confusion at ringside as the commentators, crowd and each corner struggled to understand what had happened.
After a few minutes, we went to the judges’ scorecards and Horn won 68-64, 70-63, 70-63.
“It was really disappointing to win the fight like that and I wish it could keep going,” said Horn afterwards.
“I was in front but we’re thinking about the future so not taking any more damage.”
Horn said it was the first time he’d ever had a head-clash and that he wished the fight could continue.
“It’s all in the footwork and the gap and the guy had more reach than me, so I had to keep feinting and one time when we came together (we clashed) and it sucks,” he said.
“I’m sorry to the crowd and everyone watching on TV.
“This is the first time it’s happened. I’ve not even noticed it, I can feel a little mark but it’s not affecting me.
"The corner could see it and he was niggling it so it was stopped."
Horn, who called out Anthony Mundine earlier this week, says he’s eyeing a fight in October in Auckland against a top-15 ranked fighter.
The 27-year-old former Olympian is the number one ranked Australian welterweight and now has 12 professional wins from 13 fights with one draw.
Horn was fighting on the undercard to Kiwi heavyweight Joseph Parker's fight with former Penrith Panthers junior Bowie Tupou, with Parker earning a destructive first round knock-out.
Elsewhere on the card, Dave "Brown Buttabean" Letele was rocked in an upset first round TKO loss to Kaleni Taetuli.