You are entitled to a GST input tax credit for the GST included in the price of purchases you make for use in your business. You are not entitled to a credit to the extent you use the purchases for private purposes or, in many cases, to make input taxed supplies. You will need to have a tax invoice to claim a GST credit (except for purchases with a GST-inclusive price of A$82.50 or less, although you should have some documentary evidence to support these claims).