You will only be able to use your Medicare PKI to log in until early 2018. Why don't you sign up for a MyMater ID now?
Once you have created a MyMater ID, in future you can log in via https://doctorportal.mater.org.au and you will no longer need a Medicare PKI. If you access this via https://portal.mater.org.au it will continue to present you with this screen.
