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Brown said on the Breakdown programme on Sky TV last night that he had thought long and hard about his future over the past few days.
He had been mentioned as a possible coaching assistant for prospective All Black coaches Ian Foster and Scott Robertson.
But he said his connection with Joseph, with whom he played then coached with at the Highlanders, winning a Super Rugby title in 2015, was so strong that he would commit only to coaching with Joseph in the future.
He ruled out teaming up with Foster and Robertson.
Brown said he loved coaching with Joseph and had decided to stick with him. He said the relationship between the two, how they coached and how they knew each other was what he loved about coaching the game.
"Wherever he goes, I'll go, but I'm not sure where that is," he said.
Joseph had coached Japan, with Brown, and helped the side make the World Cup quarterfinals.
Brown said they both wanted to coach the All Blacks. Whether it was the right time now, or in four years, was not clear, he said.
Joseph is off contract with Japan.
Brown admitted it was a massive decision to commit to Joseph.