O'Sullivan, of Dunedin, won the poetry category and $10,000 for his book Us, then at the New Zealand Post Book Awards in Wellington on Wednesday night.
The win was ''gratifying'', O'Sullivan said from Wellington yesterday.
''You put a lot of effort into a book so when something like this happens it's quite a pleasant occasion.''
The recognition gave ''a confirmation you're not wasting your time and confirms poetry does still have some relevance in our community''.
The judges - Miriama Kamo, Dick Frizzell, Kim Hill, Elizabeth Smither and Peter Simpson - said O'Sullivan's work got ''better and better''.
''There are poems in Us, then that leave something on the air that feels like human longing, the brevity of life, its incongruities and fleeting beauty,'' the judges said.
''There's one about seeing someone off a cruise ship and just wondering about their lives. There are several poems with geographic references to places like beaches on the peninsula, to the harbour.''
Dunedin's ''literary past'' justified its inclusion as a Unesco City of Literature.
Dunedin City Council arts and culture group manager Bernie Hawke said an application had been submitted to designate the city a literary centre, alongside Edinburgh, Melbourne, Iowa City, Dublin, Reykjavik, Norwich and Krakow.
The outcome of the bid would be announced on November 30.
Mr Hawke said the council would have ''another crack'' if the bid was unsuccessful.
