Hi,
After you have left the UK, you will need to complete the below form, to declare your pensions in the UK.
Double Taxation: UK-Australia (SI 2003 Number 3199) (Form Australia-Individual 2003)
The completed and signed form should be send to the Australian tax office for validation. They will return the validated form to you, so you can forward to HMRC, using the address on the front page of the form.
Please note for the future that this form will need to be completed everytime you commence a new pension, including state pension.
Thank you.