Marshall has thrown the considerable weight of his rugby reputation behind his former adopted country and tipped them for glory against the current World Champions - even donning a Wales first-team jersey at the Millennium Stadium in a show of support - and he says the All Blacks will be 'unsettled' by the prospect of the imminent arrival of Warren Gatland's men.
The 81-times capped New Zealander spent two seasons (2006-2008) in Wales with the Ospreys and knows all about the talent available to Gatland and he predicts both Southern Hemisphere superpowers will be worried whilst insisting Wales have every right to be in confident mood.
Wales can put players out there that are key decision makers, people that can change a game and they are more than capable of beating any of the teams put in front of them this summer. They need to go out there and believe.
"This is a great South African team, they are the current World Champions and we all know what pride they take in that and in the way that they play, but here in Cardiff, Wales are more than capable of beating the Springboks," said Marshall.
"And the All Blacks won't be super confident either, they will be slightly unsettled. They are going to need to make some changes within their set up, because they need to build towards the World Cup as well and they won't have played a lot of rugby together either.
"If Wales can go to New Zealand on the back of a win against South Africa, that will mean a lot.
"On their day Wales are capable of beating any Southern Hemisphere team and any team in the north. They have a great balance to their side at the moment and with one or two players back from injury since the Six Nations they look stronger.
"What they need to do is believe in that and take that into the first game and grow some confidence before going away to New Zealand.
"That will give them a chance to go out to New Zealand and win out there, and that will also be key to whether they can win in the World Cup."
Marshall will be the official guest speaker in the Millennium Stadium hospitality suites for the Springboks' visit and the WRU have given him a Wales jersey to wear with pride and to show where his allegiance lies on match-day.
And, although Marshall stopped short of predicting a victory for Gatland's Wales against his home nation, he did offer some words of advice to his fellow countryman as well as willing him to victory over the Boks.
"Putting a national jersey on, whichever country it is, never fails to fill me with pride and that's what the players in Wales need to hold onto when they go out onto the field against South Africa," he said.
"It's nice for me to wear the red, but this jersey is going to mean so much to the players. They need to treat this game like any other and if they can do that they can beat anyone on their day.
"I do think they can be a bit more expansive in their game plan and even throw a bit more caution to the wind and put on a show, maybe not be so conservative at the outset and they will scare teams."
Marshall also believes that, regardless of results, travelling to New Zealand in World Cup year after facing the World Champions at Millennium Stadium is ideal preparation for the 2011 tournament.
"They are going to places they are likely to revisit - the second Test in Hamilton is at a World Cup venue - and they need to familiarise themselves with everything from the pitch to the crowd, the people and the atmosphere," added Marshall.
"If they can't go out there in that environment and perform then they will know they have a lot of work to do before next year.
"So it's a great opportunity for Wales to be able to play here at the Millennium Stadium in the afternoon against the World Champions and then go onto New Zealand, where everyone loves to tour.
"It will be a real highlight for those players and they will be enthusiastic about getting out there but first doing the business here against South Africa.
"I think it's going to be a wonderful extravaganza of a game of rugby and to play in front of your home crowd before you get on the plane to New Zealand is going to be something the players are really looking forward to."
Justin Marshall is the WRU's official guest speaker for hospitality customers and will form part of the £139 per person 'Springbox Suite' package. Act fast to secure the hottest tickets in town complete with all the VIP trimmings via the official website www.millenniumstadium.com or call the WRU hotline: 08443 722 722 - other packages are also available in limited supply.