This paper by Murray Gregory, published in the
Journal of the Royal Society of New
Zealand (1991) is a survey of plastic littler
and debris on New Zealand beaches (PDF, 2.9
MB).
More suggestions and sources
Crowe, Andrew. The life-size guide to the
New Zealand beach: featuring the odd things that we
get washed up on the sand. Auckland: Penguin,
2004.
Evans, Alice. Mount Camel calling
Auckland: Hodder and Stoughton, 1981.
Hall-Jones, John. Stewart Island
explored. Invercargill: Craig Printing
Company, 1994.
Hayward, Bruce W. ‘A load of old rubbish:
Kawerua beach litter surveys, 1974–1997’
Tane 37 (1999): 15–26.
Walrond, Carl. ‘Beachcombers and castaways.’
New Zealand Geographic 56 (March–April
2002): 102–109.