So you just bought a computer and you want to know if you can claim the GST back if you’re not already registered? Well the good news is that you can back-date your GST registration. You will then be able to claim back any GST you paid in that period.

But you will also be liable to remit 1/11th of any income received in the back-dated period.
You should seek professional accounting advice when dealing with GST timing issues, as there may be other things you need to consider when choosing the time to be registered from.