"The core of that wonderfully talented 2015 World Cup squad is gone. The next generation isn't anywhere near as potent."
Keohane points to the All Blacks' recent form against France as the reason for their supposed decline.
"The All Blacks were woeful for an hour against a very limited French team in Auckland.
"In last weekend's second Test, the All Blacks were even worse. They had the advantage of an extra player from the 12th minute after French fullback Benjamin Fall was red carded. They led 21-6 at half-time but lost the second half 7-5."
Keohane believes South Africa's forward pack will be the point of difference when the two sides face off later this year.
"Erasmus is building a pack of forwards that will be the envy of every coach. He is also, thanks to the inclusion of Lions head coach Swys de Bruin, adding an attacking dimension to South Africa's back play.
"South Africa, on balance, have more depth and quality in their forwards than New Zealand.
"I'm picking South Africa and Australia to be the top dogs in this year's Rugby Championship, with New Zealand in the unfamiliar third place.
"The All Blacks aren't as good as many think and the Springboks and Wallabies are better than most think."
The Springboks will look to sweep a struggling England in their third test in Cape Town this weekend.
The 2018 Rugby Championship starts in August.