Mellett, who lives in Remuera with his wife Cathy and sons Michael and Matthew, believed the Springboks were well placed going into this tournament, with a good balance of youth and experience in the side.
Despite Sunday morning's remarkable upset to Japan, he said this feeling had not changed.
"It's not an absolute train smash, as if they've come away with zippo out of it, and now they've just got to go and win their last three games, which they're obviously very capable of doing."
He said the Springboks were "abysmal on the day", their discipline woeful, decision-making poor and he even went as far as to call them arrogant.
"One of the South African rugby writers, Gavin Rich, he actually was quoted as saying he thought the 'Boks were arrogant' and I'd go along with that."
Mellett said although South Africa contributed to their own demise, he could not ignore Japan's performance.
"I won't deny the fact that the Japanese played fantastic rugby. It is a huge highlight of that nation's history and well done Eddie Jones because he masterminded it, no doubt.
"What this has done is it's created an enormous amount of self-belief in any of the second-tier nations at that tournament, or the lower sides, that they can topple the big guys over."
While there appear to be plenty of disgruntled Springbok fans back in South Africa, Mellett said, there are also quite a few people, like him, willing to get behind the team more than ever.
"You don't just ditch something overnight. I think it's a case of watch out now because these guys are on the comeback."
He said while some of his South African countrymen and women who now are living in New Zealand eventually change their rugby loyalties over time, he never could nor would.
"I think if you're not so much into it, it's a lot easier to actually say, 'Oh we'll just put on a black jersey now'," he said.
"If you have been so passionately involved with it over the years, it's difficult. Particularly when the All Blacks have always been, from a rugby perspective, our absolute arch-rivals."
Cup super fans
Today: South Africa (Springboks)
Will play: (Pool B) Japan (lost 32-34), Samoa, Scotland, USA
Ranked: Third.
Team logo: The South African green and gold 2015 RWC jersey now has the Protea (national flower) as the main emblem, with the traditional Springbok logo on the sleeve.
Anthem: Nkosi Sikelel' iAfrika
2011 RWC result: Lost 11-9 in the quarter-finals to Australia, after finishing top of their pool.
Rugby World Cup record: Won 1995, 2007; Third 1999; Quarter-finalist 2003, 2011.
Nkosi Sikelel' iAfrika
2011 RWC result: Lost 11-9 in the quarter-finals to Australia, after finishing top of their pool.
Rugby World Cup record: Won 1995, 2007; Third 1999; Quarter-finalist 2003, 2011.
