The following web site suggests that C. bonariensis has toothed leaves and larger flowers, and C. canadensis has entire leaves and smaller flowers. Othe Conyza also described:

http://weeds.brisbane.qld.gov.au/weeds/flaxleaf-fleabane

Hi

In this article there is a table with distinguishing features of three Conyza species.

Best wishes

Hi

For the size of the involucre and number flowers; in C. bonariensis  involucre 4-6 mm long, hispid and flowers very numerous (up to about 250 or more).

In C. canadensis the involucre  3-4.5 mm, glabrous and flowers usually 40-60.

Note that Conyza is now included in Erigeron.

A key to and profiles of the (mostly naturalised) species in Victoria, Australia which includes E. canadensis and E. bonariense, plus several other similar species can be found by following the link in the on-line Flora of Victora, VicFlora

http://data.rbg.vic.gov.au/vicflora/flora/taxon/03ffa7ad-0b4d-442b-888a-ea47370f2896

Neville - thanks - hadn't caught up with this in New Zealand yet. Cheers