If someone is share trading as a business in their personal name, is it a must for them to register an ABN? If someone had an ABN from 10 years ago which has been deactivated, is it be possible to re-activate the same ABN from a date in the past? If so, is there a time limit as to how far back you can backdate the ABN? Eg. 1 month, 1 year, etc.