I am currently on a Telstra 50/20 plan (HFC) paying more than I should. With the recent deals I was looking at cancelling my Telstra service so I could move to ABB. While exploring their website around service cancellation, I clicked through to the various reasons for cancellation and found a notification on the "I've had a poor experience" link that offers $20/mth off for 6 months to stay with Telstra NBN (See screenshot)
I don't know if this is targeted or only applicable to specific plan speeds although it will take 2 minutes to check so worth seeing if this works for you. For reference I have been with Telstra for more than a year.
The deal link requires you to be signed in to My Telstra. To navigate there, sign into My Telstra via telstra.com.au. Select your home internet service under "Account Overview" and then click on "View or manage your plan". Then click on "Cancel your service" and then click on "I've had a poor experience". You should then see the message bar as per the screenshot near the top of the screen.
Once you click on the banner you will see a page that says:
"Your $20 monthly credit on your Telstra nbn service will be valid for 6 months and will be applied within 2 business days.
The credit will appear on your bill as ‘Bonus Credit’ within 2 bill cycles. If your nbn service is cancelled within 6 months of receiving your credit, the credit will be removed."
Good luck!
EDIT: It seems you might need to click through some of the options for cancellation before this appears. I had clicked a couple of options before seeing it pop up.
Definitely worth doing, I do this about twice a year with Audible, go to quit and get 3 months at half price