The Mosses of New Zealand by Jessica Beever, K.W. Allison and John Child, pub.1992 University of Otago Press
Another book that is worth having is Mosses and Other Bryophytes - An Illustrated Glossary by B and N Malcolm, pub. 2000 Micro-Optics Press. Full of botanical definitions used to describe the bryophytes, plus lots of beautiful close-up photos to show you all the bits and pieces and to explain the meanings of all those botanical descriptions.
Unfortunately there is nothing available at the moment specifically on liverworts, unless you manage to pick up a second-hand copy of The Liverworts of New Zealand" by K.W. Allison and John Child, pub. 1975 University of Otago Press.