That may be your thoughts but it is not supported by the rental booklet or the tax ruling. FWIW they were my thoughts before investigating it.
I doubt a private ruling will be necessary - it must be a relatively common and well established principal.
I think that paragraphs 14,15 and 16 of IT 2167 are relevant to your sutuation
Para 14 and 15 state that rent collected from a relative would represent assessable income but that not all losses and outgoings would be deductable. It depends upon what the main purpose of the taxpayer in acquiring the property was ie to provide a relative with a good home at a moderate cost or to earn assessable income.
Paragraph 16 gives us a better clue to your answer : "as a working rule, income tax deductions for losses and outgoings incurred in connection with the rented property may be allowed up to the amount of rent received. Whether any additional deduction is to be allowed will depend upon the nature of any further information provided by the taxpayer. "
I think it says claim deductions up to the amount received and have a good reason to claim more. This ruling is old (1985) - pre self assessment, so maybe a private ruling would be better?