PPOR into Investment – Tax Issues???

I currently own a townhouse which I had bought for $290k in 2008 and the borrowing on it was $230k. Now we want to upgrade and move into a bigger house. We have some savings in our offset account and the value of our townhouse has also gone up to $370k. Also I’ve been making some extra repayments towards our home loan and the current borrowings (Principal) has gone down to $215k. I want to refinance the townhouse and use the equity to buy our new home (new PPOR) and make our current PPOR into Investment.

But if I make my current PPOR to Investment then what will be the tax implication in terms of interest?

On which amount the tax benefit (interest) will be calculated for the Investment Property i.e:

1. On the Current Value of the Property – $370k.
2. On the Original Buying Price of the Property – $290k.
3. On the Original Borrowings – $230k.
4. On the Current Borrowings -$215k
5. On the New Borrowings after the refinance – say $330k.

It is really confusing and my all my future plans depend upon how much can I withdraw from my current PPOR to maximise our tax benefits and our contribution towards the new PPOR.

Thanks in advance.