If you redraw money then you still have one loan with two purposes. The analogy I know of is that once you piss in a bucket of milk it is very difficult to un-mix the two liquids!

Say you have a PPOR loan of $500k and you pay down $100k and then redraw that $100k and buy $100k worth of shares.

You now have a $500k loan where you can apportion 80% of the interest as non-deductible and 20% as deductible. Sounds easy right?

After one month you get charged $2000 interest (balance $502,000) and make a $3300 repayment (Balance $498,700). After another month you get charged $1995 interest and make a $3,300 repayment. (Balance $497,395). Still with me?

You could easily split the interest as 20% deductible and 80% non-dedcutible. But could you split and keep track of the principle repayments in an 80/20 manner? Yes, with good record keeping.

Fast forward a year and your loan balance might be down to $485,000. Now you redraw another $15,000. You get charged $2,000 interest. How much of this $2000 interest is deductible now? What if you had 12 redraws over the year (trying to implement your debt recycling strategy)...

Can you see how this starts to get very complicated very quickly?