Five minute bird counts are on of the more imprecise monitoring methods. For most species it is a waste of time and effort. Counts vary with observer, time of day, season etc. so all you get for a multi-species count is a reliable bird list - the numbers mean nothing.
Graeme Elliot has looked at a couple of species and it seems to work for bellbirds and mohua. But my experience with most of the other native passerines and tui show far more reliable counts with transect distance sampling.
If you are steering volunteers to bird counts, you'd be better off using Eric Spurr's Garden Bird Survey format.
http://www.landcareresearch.co.nz/research/biocons/gardenbird/