New York-based KKR said it was the firm's first infrastructure investment in New Zealand.
The Ritchie family will continue to hold a stake in the 86 year old company, with Andrew Ritchie to be appointed chief executive, as managing director Glenn Ritchie retires.
KKR in a statement said it saw continued demand "for high quality, greener public transport solutions in New Zealand".
"Buses account for 75 per cent of total public transport trips made in the country and play a critical role in connecting people to places," it said.
"Our investment also reiterates KKR's strong commitment to investing in critical infrastructure assets in New Zealand and globally through our expanding portfolio."
The NYSE-listed investor said it looked forward to working closely with the Ritchie family and in partnership with the Government to deliver safe, reliable and sustainable public transport services.
KKR was making the investment from its Asia Pacific Infrastructure Fund.
Founded in 1976 by Henry Kravis and George Roberts, the firm had US$429 billion ($617b) of assets under management as at June 30. It has 109 portfolio companies in its private equity funds. They generated US$244b in annual revenues as at March this year.
The deal was subject to customary closing conditions and OIO approval, which was expected within four to five months.