To resolve this issue you can download this file:

(Outdated) SSiNet SSL Security Certificate Installer

You may also install the certificate manually. In both cases you will need to restart your browser so it can load the certifcate.

To install the SSL Certificate manually follow these steps:

Open the certificate: ca.crt

Open the certificate file

Allow the above to run

Select "Install Certificate" make sure it is issued to: "SSiNet Network Communications"

Select "Next" in the Certificate Import Wizard window

Select "Place all certificates in the following store" and select "Browse"

Select "Trusted Root Certification Authorities" and click "OK"

The Certificate store should say: "Trusted Root Certification Authorities", select "Next"

Compelete the Import Wizard and click "Finish"

A security warning will appear;
   verify "SSiNet Network Communications" as highlighted in the picture above and select "Yes"

Click "OK" and restart your browser. That's it, your done!