Yes you do before you go.  Can do online or ring up.

When to tell us about your travel

You should tell us you are leaving Australia if you:

are going to live in another countrywill be away for more than 6 weeksare paid under an international social security agreement, orreturned to live in Australia within the last 2 years and you started being paid Age Pension during this period

Otherwise you do not need to tell us that you are leaving Australia. Australia's immigration department will advise us when you leave Australia and when you return.

The easiest way to tell us about your travel plans is to use your Centrelink online account through myGov.

You can get tailored information on how your payments and concession cards could be affected.  

Your rate of Age Pension generally will not change if you travel outside Australia temporarily for less than 6 weeks.

https://www.humanservices.gov.au/customer/enablers/age-pension-while-travelling-outside-australia