Why did you not upload the tracking number? If you didn't, then the buyer is going to think it's been sent untracked, so know they will win a not received dispute. 

Create a My Post account on the Aust Post website. It's free. That should give you access to My Post Business as well (despite the name, you don't have to be a business). Request a My Post card. Install the My Post app on your phone or other device.

When you take a parcel to the PO, scan the card when paying the postage, or getting a lodgement scan if you purchase post online. That automatically adds the tracking to the app. I then go into the app and rename the tracking with the buyer's username, so I know who owns what tracking number. If you don't have a device, you can see the tracking on the website as long as you've scanned the card.

If you have a printer, print postage labels from My Post Business (make sure it's Business, and not just regular My Post). If you post over a certain amount you can get postage discounts. Depending on the amount you post, depends on the discount. If you are high volume, the discounts can be significant. You can link the account to your ebay account, so the tracking is automatically uploaded to ebay.

If you prefer to post and pay at the PO, have a box or something to put your receipts in. Then you know where they are. Write the buyer's username on the receipt and add to the box. Keep each receipt for 6 months. When I was doing that, I had baggies for each month to put the receipts in. That way I didn't have to go through the mountain of receipts to find the ones over 6 months old.

If you didn't upload the tracking number and you can't find the receipt, then unfortunately, you have no choice but to refund. If the buyer has opened a dispute, and they close it before you refund, you will get a defect on your account.