I received a letter from TPG Billing Team <firstname.lastname@example.org> telling me that my credit card could not be authorised and service could be interrupted. I checked my bank account and tpg account and found that everything is in order and money were properly taken from my credit card to TPG. I think that this letter is a spam or phishing letter, but would like to confirm it.
Unofficially: I don't think the TPG Billing Team would be using a Telstra email address. If it was an email, what was it asking you to do? As long as you didn't click on any links in the email, you should be safe.