Donald was a teacher at the Faculty of Law, University of Auckland, from 1968, taking early retirement as an Associate Professor in December 1998 after 31 years. During that time, he taught 'Land Law', 'Vendor and Purchaser', and, for a time, 'Equity and Succession'.
From the late 1970s Donald has done opinion work in private practice and since leaving the University has practiced as a barrister doing opinion work in the areas of 'Land Law' and 'Vendor and Purchaser'. He has also acted as arbitrator in some matters, and has assisted other barristers in some litigation work.In the mid-1970s Donald became involved as a co-author in the writing of "Land Law in New Zealand". That work is now available on-line and in loose-leaf format. It is one of the principal texts on land law in New Zealand today. In 2010 the re-publication of this work in hard copy in projected series of texts began again with a volume on Easements, Covenants and Licences.
In 1992, Donald began work on a text on contracts for the sale and purchase of land in New Zealand a subject he had taught since 1975. The first edition of Sale of Land was published in 1994, followed by the second edition in 2000. The third edition is in the course of preparation and will be available in 2011. Donald publishes the work personally.