Since Taskers are independent contractors, you’ll have to request a tax invoice from them. As the provider of the service, they should be able to get you one.
If the task has already closed and you weren't able to get the Tasker’s contact information, you can still get in touch with them via Airtasker Private Messaging. This feature remains enabled for the next 7 days after the task completion. For help with requesting an invoice after this time, please contact us to assist.
You can also include a tax invoice as a requirement in any future task that you'll post.
Alternatively, if a simple receipt of the task payment is enough, you can find this in your account under Payment History.