1. NSW Department of Industry and Investment, Tamworth Agricultural Institute, 4 Marsden Park Road Calala, NSW 2340, Australia.
2. School of Science and Technology, University of New England, Armidale, NSW 2351, Australia.
3. Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
4. Corresponding author. Phone: +6-2-67631119. Fax: +61-2-67631222. E-mail: mike.sissons@dpi.nsw.gov.au