It’s usually optional to supply a TFN, but if you don’t CommSec will deduct tax from any dividends your shares earn at the top marginal rate, though you can claim the excess back in your tax return if you submit it. If you supply a TFN, CommSec will not deduct tax but you must declare your dividends in your tax return and you will be taxed on them then at the applicable rates that apply to you subject to franking credit adjustments which you should read up on.
