Prepaid interest

I am thinking of prepaying interest on my investment loan to reduce tax for 2012-13 as may have a lower taxable income next financial year. I have a lump sum of money in offset account,which would cover the prepaid interest payment, but would like to preserve this possibly going to buy a PPOR in the near future. Do you think the tax department would have a problem with borrowing money to pay the prepaid interest (e.g. from an unused line of credit account that I have in place) or via redraw facility from the investment loan – to my mind, both these options would involve compounding interest, which I understand is an "iffy" area in terms of claiming tax deduction if the main reason is to reduce tax. Prepaying the interest also allows me to take advantage of a discounted rate, however (4.86%) for 1 year fixed with CBA. Any ideas or thoughts on this arrangement, or would I be safer to pay the interest amount with money from the offset account?    Thanks w8up