I cannot get https://www.business-trader.com.au to return. It just goes off into never never land which is a bad thing regardless of your intent.
I would take the warning from Google to heart. This is an indication of a problem (or at least a potential problem). Most people don't get the fact that improperly implementing HTTPS pages on a site can lead to the same URL canonicalization issues that www vs. non-www cause.
If your site doesn't use HTTPS then you should have 301 redirects in place such than when someone requests a URL with HTTPS that it gets 301 redirected to the HTTP version of the URL.
If you site DOES use HTTPS then you should know exactly which pages require HTTPS. Requests for all non-HTTPS pages with HTTPS as the protocol should be 301 redirected to the HTTP version of the URL. Requests for all HTTPS pages with HTTP as the protocol should be 301 redirected to the HTTPS version of the URL.
And if your SSL certificate is for a different domain (say, https://secure.example.com/) than the actual links on your site (which may have https://www.example.com/) then this can lead to all kinds of issues with browsers... which if nothing else is a VERY bad user experience.
Find the problem and fix it.
Here;s what I get when i try to access https://www.business-trader.com.au/
Internet Explorer cannot display the webpage
Most likely causes:
You are not connected to the Internet.
The website is encountering problems.
There might be a typing error in the address.
http://www.business-trader.com.au/ however loads just fine. Are you sure you have an SSL certificate to be able to load an https:// page off of your server?