When you are making a purchase the final cost will come along with a GST amount. You can claim the input tax credit on the GST paid on the purchases that you make. So when you are purchasing machinery for your business, you would pay GST in addition to its original price. This GST can be claimed as a credit in the same way as inputs. But you cannot claim an input tax credit if you claim depreciation on the GST paid while purchasing the capital asset.  

Inputs are consumed while the final products are made and are treated as business expenses. But the capital goods(machinery) are not consumed when the final product is made. They cannot be deducted as business expenses in the year of their purchase, but instead, they can be depreciated over the course of their lives. Businesses will recognize part of the cost each year through accounting techniques.