It appears they've changed the way Acacia's are classified, some have retained the name 'Acacia' for Australian species only and used other names for non-Australian species, whereas some have ignored the new convention and retained the old classification! (Wish they'd sort this type of crap out on a world wide basis and achieve consensus!)
Anyway, the following websites may assist you - the first one asks for input data regarding the plant so you will be required to do some measurements with a simple rule, and take a few other observations.
webpage
webpage
The species could be non-Australian so, failing the above searches, it may be worthwhile emailing the Australian National Botanic Gardens with a series of photos - they may actually assist you regardless of the plants origin.
Email:
anbg-info@anbg.gov.au
Happy hunting.