If you do down the track decide to make the property an investment property you can take the money out of the offset and the full loan amount is tax deductable.
If it's in the redraw and you take it out you cannot deductable.
As stated by myself on this forum many times - you CAN use a redraw and claim the interest on the loan as tax deduction if you borrow that money for an investment property (or use previous residental home as IP) It makes no difference for tax deductiblity purposes - yes, I have done this with my previous mortgage and yes, my tax was done by an accountant and no, ATO had no issue whatsoever with this.