WHEN Greater Western Sydney ruckman Rory Lobb discusses how physically demanding AFL is, he speaks with a level of authority that most players lack.

Lobb will play a crucial role in Saturday's preliminary final at the MCG, where the Giants must defeat Richmond to progress to their maiden Grand final.

The 24-year-old handled the bulk of the ruck duties for GWS in their semi-final against West Coast, filling in for injured teammate Shane Mumford.

It is a taxing position.

Even for Lobb, who left school in Year 9 because of relentless bullying and started fly-in fly-out (FIFO) work in the Pilbara and Darwin.

"It was a lot different to being at school. I had to grow up pretty fast," Lobb said.

"I was doing a painting apprenticeship but also labouring, building houses and that sort of stuff.

"Sometimes two weeks on, one week off."

It was hard work for a 14-year-old adjusting to life away from home with limited help from colleagues twice his age.

The laborious days have given the big man - who had set his sights on a basketball career as a junior but now prides himself on crashing packs and plucking contested marks - a sense of perspective that few players have.

But, as Lobb points out, football also has its physical challenges.

"People don't realise how hard the footy world is. You put your body through hell," he said.

"AFL is very hard on your body. A lot harder than the workforce I was in.

"Just to play weekends, it's very hard."

Lobb, who missed five games this year because of a groin injury, is ready for his ruck showdown with Tiger Toby Nankervis.

The West Australian, who has stacked on 15 kilograms since joining the Giants after an eye-catching WAFL season in 2013, is still learning on the job in the absence of Mumford.

"It's totally different when you're first ruck," Lobb said, reflecting on his 39-hitout performance against West Coast last week.

"Being around the ball the whole time and getting that connection with the mids, obviously I barely went forward against the Eagles."