If you are going to get a monthly pension, consider keeping a percentage equal to your highest tax bracket aside.

At the end of your first Australian tax year, you may have to pay some tax on it. The amount depends on which tax bracket it will sit in, if you have stopped paying tax to the UK and your 1st year tax refund amount. 

In my 1st year I put aside 37% a month incase my tax refund wasn’t sufficient to cover it and a debt was raised.

After my first tax year, ATO got in touch and a provided me a weirdly excessive invoice for how much I have to pay quarterly. In advance!  I continue to put enough of my pension aside every month to meet the ATO’s quarterly bill. 

(Factual, informational only, not a recommendation)