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The 44-year-old is part of the coaching staff at John McGlashan College this season.
He will help head coach Nick Anderson with the First XV.
Brown, who coached Otago from 2015 to 2017, has been in Japan coaching Sanix in the Top League.
The 2020 season was cancelled due to Covid-19 but Brown had planned to be back in New Zealand from mid-May until August-September anyway.
"It is more about helping them out, really," Brown said.
"Johnnies were looking for a coach and asked me for a few names. I gave out a few names and just said I could help them out."
Johnnies open their campaign against Southland Boys’ High School in Dunedin on Saturday.
They both shape as title contenders alongside the likes of Otago Boys’ High School and King’s High School.
King’s has been widely dubbed as the team to watch this season. It has retained a large core of last year’s side.
Brown has been with Johnnies for about three weeks, although he did a little bit of work with the team prior to lockdown.
"I think they are going to go OK. They’re going to be competitive. I don’t think they are going to be as big size-wise as Otago Boys’, King’s or maybe Southland Boys’.
"But there are some guys with a lot of potential, a lot of talent. I think we’ve got 13 year 11s, so the future at Johnnies is looking quite good.
"Maybe they’ll get that experience and stuff this year but we are certainly not there just to make up the numbers.
"I think the brand of footy that John McGlashan has played over the last few years is one of the reasons why I wanted to get involved and help them out.
"We’ve got three sets of brothers in our forward pack, which is quite unusual and quite cool as well. They’ll enjoy playing with each other and playing for each other.
"That is all I want them to do, really."