Evolution buys 11% of hospital rival EVOLUTION Healthcare, an Australian private equity healthcare investor that owns Lower Hutt's Boulcott Hospital in New Zealand, has acquired a potential blocking stake in Acurity Health Group, the owner of three rival private hospitals. Sydney-based Evolution has built an 11 per cent stake in publicly-listed Acurity, according to a substantial security holder notice to the NZX on Wednesday. Evolution has built the stake at $NZ5.50 a share since Friday, a 36 per cent premium to Thursday's $4.05 closing price. Evolution is increasing its exposure to the private health sector as ageing populations in Australia and New Zealand put increasing demand on public health services, with a likely spinoff for private hospitals. The company reportedly paid more than $NZ20 million ($A17.62 million) for Boulcott Hospital after being granted Overseas Investment Office approval in November last year, and the stake in Acurity has cost it about $NZ10.5m. Under New Zealand takeover rules, a 10 per cent stake is enough to prevent a full takeover of a company and may be enough to request a board seat. Evolution managing director Ben Thynne didn't immediately return calls requesting comment. Acurity owns Wakefield Hospital and Bowen Hospital in Wellington and Royston Hospital in Hawke's Bay. It is 50.01 per cent owned by Austron, a venture between the Royston Hospital Trust Board and interests associated with Canterbury businessman Mark Stewart, who jointly acquired the controlling stake for $NZ6 a share last year. Evolution has said it plans to expand Boulcott Hospital, and in Australia the company owns Shellharbour Private Hospital, mental health facility South Coast Private and Canberra Private Hospital.