Hi briggsn.
Another user is getting the same error but only when logging off from Post Office, and Android device was mentioned. My comments were in that context.
I think this is the sequence of events on your Android device.
You open the browser and connect to www.tpg.com.au. This gives you a page with two tabs to logon to Post Office or My Account. Tap the padlock symbol next to https in URL. This opens a window showing security information. On my Android device, it says I am using "TLS 1.2".
Logon to Post Office. This takes you to new-postoffice.tpg.com.au. Tap the padlock again; it shows "TLS 1.2" again.
Logoff from Post Office. This takes you to postoffice.tpg.com.au. Note there is no tab for My Account; a different page from the logon screen. Tap the padlock; it now shows "TLS 1.0", which is obsolete in favour of TLS 1.2 and 1.3. On your device, tap the "Advanced" button for more info.
Different versions of Android and Chrome must show this info differently. Yours pops up this error message; my Android v4 and v5 aren't as obvious.
Don't know why it should be happening now after all this time. The logging off is done with TLS 1.2 but if you logon again from the page that has just been displayed, the security is not as good as logging in from the original logon page.
TPG should change the Post Office app so that a logoff redisplays the logon page with the two tabs.
