I noticed one of my express post parcels sent 15/10, had not been received by the buyer by the estimated delivery date of 27/10.

It had been sitting at Somerton VIC. with the message "Delayed – waiting to be processed for delivery", since the 22/10.

I know express post is taking longer at the moment, but this seemed ridiculous.

So I immediately lodged an enquiry with Australia Post, and lo and behold, it prompted delivery the very next day.

Would recommend lodging an enquiry with Auspost yourself, shows you were trying to help should the buyer try to get a refund via a dispute.

Don't refund her, you've done nothing wrong, and ebay will close the case in your favour.