I have had this happen once or twice. The clock is just completely out. It says the lot has ended, but then it also says there are XX minutes remaining ... but you can't bid!!
The quick solution is to open the lot using a different browser. So, if using Internet Explorer, try using Google Chrome, etc.
The longer solution is to delete all of your cookies and cache - either for the entire computer (which is a hassle), or more specifically for any and all sites that include ebay.com.au in the URL.