If i have it the right way around you would move into the new house and rent out your current PPOR.

Only problem you have there is that the interest on the new loan will not be tax deductble whilst your current townhouse will be positvely geared and the net rent after expenses together with the income you receive from your father will be added to your income and taxable at your highest marginal rate.

Depending on the actual figures you could look at selling your current Townhouse to a Trust structure , borrowing 100% of the current market value and use the net loan after repayment of the $70,000 as deposit on your new property.

This converts the non tax deductible debt into tax deductible interest although as mentioned all of the information is required to make a worthwhile comment.

Stamp duty will be payable but again depending on how long you intend to stay in the new home will determine whether the exercise is worth it.