Where can I post my business purchase transactions?

Hi Athula, 

Thank you for reaching out on this. 

This question would best be answered by your accounting professions. Here I found an article from ATO's website for your reference: 

Business tax deductions

https://www.ato.gov.au/business/income-and-deductions-for-business/deductions/

If you are looking for how to enter them as an expense, please refer to this article from our community:

How to enter, edit, or delete expenses

https://quickbooks.intuit.com/community/Income-and-expenses/How-to-enter-edit-or-delete-expenses/td-p/187002

Hope these help and please let us know if you have any questions. Thank you.