Not entirely sure about eBay Australia, but on ebay.com when an eBay gift card is used the first time, any remaining balance is linked to the eBay account used, and the balance appears at checkout with a box to check to apply it to a future purchase.

The eBay Gift Card FAQ on ebay.com.au is not entirely clear on this point. One part says that the remaining balance remains on the card, while another says that the balance is saved against your account and you must use the same eBay account.