Hundreds of people visited the library on Saturday to celebrate the milestone.
The library was founded by the Rev John Albert Fenton and Emily Orbell in the West Hawkesbury (as Waikouaiti was known) Athenaeum Hall in 1862.
The sesquicentennial was initially planned for last year, but was postponed after the sudden death of a staff member, Dunedin Public Libraries marketing co-ordinator Kay Mercer said.
There were historic photographs, library cards and other memorabilia on display and a chronological, handwritten record of all books received by the library.
There was also entertainment and a smorgasbord of local cheeses.