ABN (and GST registrations can be backed dated) and the registration is taken from the date you state on the application - not the date of registration.
As for someone classing themself as a trader for this current year applying for an ABN now has little bearing on being able to do that.
There are many other factors that need to be considered and if the tests as to if you are conducting a business (as a sharetrader) are passed and have been for the full year the date of the ABN application is irrelevant
