If Kieren Jack put on a few extra kilograms, his dad reckons he could slide right into an NRL side.

But Garry Jack, a rugby league great for Balmain, NSW and Australia, is sure his son will finish his career as an AFL player.

Swans co-captain Jack told Triple M Radio earlier this month he would consider switching football codes if the opportunity arose, as the challenge comes with great reward.

Garry says these comments were off the cuff, but still thinks his son would be able to make the switch successfully.

He said the 27-year-old would find it easier than the big-name rugby league and union players who have hopped codes the other way in recent years.

“He played league until he was 14, so he’d had a lot of league beforehand,” Jack told AAP.

“He understands the game a lot more than Israel Folau and Karmichael Hunt understood AFL.”

The Swans midfielder would first need a decent spell in the gym, according to his dad.

“He’d have to put on some weight,” Jack said.

“He’s not really that big at the moment. He’d have to put on another 10 kilos.”

But Jack would be surprised if his eldest son ended up following in his goose-steps, being such an important part of the Sydney side.

“There was a chance there probably five or six years ago,” he said.

“Because he was on a year-by-year (contract) for the first three years.

“If it hadn’t worked out at Sydney he might have come across.

“But not now. I think he’s going to be AFL through and through.”

As a kid, Garry said Kieren was demanding, and was rarely seen without a ball, club or racquet in hand.

Fast forward, and Kieren Jack will line up in Saturday’s AFL grand final 25 years after his father played in the NRL equivalent for Balmain.

The older Jack looks back on it fondly, despite his side’s close loss to Canberra.

“(Kieren) was two. And his brother (Rhyce) would have been one month, so it was a pretty busy time,” Garry said.

Youngest son Brandon, who joined the Swans last year, had not yet arrived then.

Balmain lost in extra time to the Raiders after leading 12-2 at halftime.

“It was gut-wrenching. You’ve got to take your chances when they come,” Jack said.

Garry might have given Kieren this kind of fatherly advice in the past.

But since his son played in Sydney’s 2012, he doesn’t need to push his wisdom so hard any more.

“I did a couple of years ago. But he’s 27 now,” he said.

“I think he’s heard all my stories. I think he’s fed up with (them).”

Garry Jack will be at the MCG when Sydney battles Hawthorn for the cup, hoping for the same outcome as 2012, which helped make up for Balmain’s premiership loss 25 years ago.

“That sort of took away a bit of the pain from `89,” he said.

“I think you’re much happier for someone else’s enjoyment than your own.”