The 56-year-old could have coached the All Blacks.
In terms of ability, that’s a given. Unprompted, All Blacks great Christian Cullen added his name to a very long list of admirers on New Zealand television on Tuesday night, putting Rennie and Wayne Smith “right up there” as the best coaches he played under.
There is something about Rennie’s communication style that simultaneously puts players at ease and reminds them he’s the head honcho.
Few who have played under Rennie ever complain they don’t know where they stand, and they respond to that clarity. It’s better to be delivered harsh feedback straight up rather than continually wonder if the coach rates you or not.
But if Rennie delivers honesty, the trade is that players have to meet standards.
Some have already found that out the hard way. Isi Naisarani is not in New Zealand, left in Australia with a list or improvements to make that have not been made public, but assuredly are clear in private.
We can only speculate about what that might be, but here’s an anecdote that might give an insight into how Rennie works.
It’s pre-season at the Chiefs and the franchise’s top players have come back from a beginning-of-year testing camp with the All Blacks.
There’s really not much to talk about, but Rennie’s eyes light up when he reveals that Chiefs prop Nepo Laulala, on the comeback from a knee injury, had beaten former Crusaders and All Blacks hard man Owen Franks in a running test.
You get fit under Rennie, or you go out the exit door.
He will be similarly hard-headed about the commitment levels needed at the breakdown.
Under Rennie, the Chiefs developed a reputation for pushing the envelope at the breakdown.
It was all sour grapes, of course, particularly from the Crusaders, whom the Chiefs seemed to develop a particular disdain for – firing up the likes of big prop Ben Tameifuna to niggle Richie McCaw at lineouts.
Opposing teams complained that the Chiefs took players out off the ball, but what they really couldn’t handle was the level of brutality the Chiefs were prepared to bring to that area, particularly when they were trying to get quick ball for No.10 Aaron Cruden.
Those Chiefs years under Rennie, particularly in 2012 and 2013, made everyone in New Zealand raise their game in the contact zones. It is my firm view that had Rennie’s Chiefs come along, the title-winning Highlanders, Hurricanes and Crusaders sides that followed would not have hit the heights they reached.
The breakdown, therefore, is the area where you might see the first immediate signs of Rennie’s influence on Sunday.
It is too early in his tenure to expect the Wallabies’ full range of tricks, but “brutality” – a word that is firmly part of the Rennie lexicon – is something that requires no talent.
There is another aspect to Rennie’s coaching that will appeal to Wallabies observers who feel the modern player has been spoon fed too much information, thus denying them the chance to build game understanding on their own.
The Chiefs occasionally used to open the doors to training to the public and Rennie and Smith would run counter-attacking drills. Occasionally, the attack would break down through a failure to identify the space on the paddock and the coaches would halt proceedings. But a rocket wasn’t about to be delivered. Instead, it was a question.
“What made you make that decision?” Smith would ask the player at fault, trying to get him to work out the answer himself.
Rennie will coach the Wallabies, but he will also want them to think for themselves.
Paul Cully is a rugby columnist for The Sydney Morning Herald.