"(He) is disappointed to be leaving NZC at this time, as he has enjoyed his work there," the organisation said in a brief statement. "However, due to family circumstances, he has decided to return to Australia."
Buchanan had reportedly been sidelined at NZC since late last year, when he supported Ross Taylor after coach Mike Hesson sacked the batsman as skipper in favour of Brendon McCullum.
He was appointed in May 2011 with a contract running through to the 2015 World Cup, with his role seen as providing "a consistent coaching philosophy" in New Zealand, while also overseeing talent identification and the selection panel.
NZC did not reveal if Buchanan would receive a payout for the remainder of his contract, saying employment agreements were strictly confidential.
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"NZC respects John's decision to return home and we wish him well for his future endeavours," it said.
Buchanan's eight years at the helm of the Australian side coincided with a golden era for the Baggy Greens. From 1999-2007 they claimed three World Cups and won a record 16 consecutive Test victories.