It hit him directly in the middle of his chest, causing him to leap into the air and veer sharply to his right, with Pateman having no chance of remaining in the saddle.
The injury wasn't immediately apparent and Sea King, trained by Central Districts jumping supremo Kevin Myers, galloped around at the head of the field for the majority of the race, jumping the fences without a jockey.
It was only when he was caught the huge laceration in his chest became obvious.
"It is pretty graphic but luckily it looks worse than it actually is," says part-owner Rachael Frost, who herself trains horses.
"It was a terrible thing to witness from over here watching on TV, seeing that happen to him and not knowing how bad it was.
"We got a pic sent over from my partner's mother soon after and it looked terrible.
"But it is basically a flesh wound and for a horse it is worse if it is structural, like a bone in a leg."
The injury is expected to heal relatively quickly and Frost and her co-owners are relieved it was not worse, now their initial shock wore off.
"He won't be back racing this season but he should be fine to race on and the most important thing is he is all right long term," says Frost.
"We had actually been really lucky with him, he has raced over fences for so many seasons without any problems.
"So to get hurt like this, in an incident on the flat, is incredible bad luck.
"Apparently Steve was furious after the race about the rail coming loose."
Just hours after the horrific incident, Frost and her co-owners were on stage accepting their shared award for last season's jumping achievements, her news of the injury during her speech stunning the 400-plus crowd.
"It was a bittersweet day, being part of him winning such a big award but knowing what he had been through," she said.
"But the reports today are that he is comfortable and that is the main thing. It could have been so much worse."
