For the first year I used the figures that I acquired to calculate the holding costs before purchase, such as council rates, LL and building insurance, mngt fees and I deliberately left off depreciation. Bank interest is very easy - IO loans, no principal to worry about. Remember to income split the rent and only show each person's share of the expenses if they are in joint ownership. The form will prompt you.
If you over estimate your income and under estimate your costs, the ATO will be happy. For the following years I use the same figures the accountant puts into the actual tax return to do the Tax Variation.