Royalty descened on Sydney Football Stadium when the prince and his wife Meghan Markle saw the NSW Waratahs play Moana Pasifika.
Catt, assistant coach at the 'Tahs, has known Harry since he was a young child, spending time with him at the 2003 and 2007 World Cups.
The prince joined in the celebrations as he followed England's 2003 World Cup-winning side across Australia and was also present during their run to the final in France four years later.
"I've known Harry for a long time," said Catt, who played in both finals.
"He was involved with the England set-up when (coach) Clive Woodward was there, so from really early doors. I think I met him first as a five-year-old or something like that.
"And he was always involved with all of Clive Woodward's era. When we won the World Cup in 2007, he was out with us.
"He genuinely loves his rugby. I'm not sure about Meghan, but I know Harry definitely loves his rugby," he added.
"Good guy and it's great for the game, to be honest, for us to have people like him, royalty coming in, is a huge benefit for us."
Asked about his recollections of celebrating with the young prince in 2003, Catt said: "He's a good guy, he loves fun.
"It's just good for him to switch off and and be part of it (the rugby)," he added.
The couple are on a four-day Australia visit to attend business and charity events.