If I redrew, say, 300k from my PPOR loan to fund an IP and this took the total loan back up to, say, 500k - only the interest on 300k which was for the IP would be tax deductible, not that of the rest of the loan which is still for the PPOR - and/or any other redraws for non tax deductible purposes,say, a new car.
Obviously using redraw only works if the IP is worth less than the PPOR.