As Rachel has indicated, you can lodge an objection. But note (and I don't know if any of this applies to you), there are a range of circumstances that would point to a property not being genuinely available for rent including factors such as:
1) The property being blocked out for private use in many of the peak periods such as Christmas, long weekends and school holidays
2) Restrictions on renting the property being unreasonably tight, such that it prevents people from renting it (ie 2 week minimum rental for example).
3) The property being in an area where it isn't realistic that anyone would rent it for short term use
4) The property isn't advertised widely and no real concerted effort is made to maximise the periods it is rented.
Or if records such as electricity bills show that the property is being used in periods when it was marked as "available for rent but not rented", that may be a red flag to an auditor as well.
Also, if it is rented at below market rates to friends and family, your deductions are going to be limited to the amount of the rent received.
But remember, any holding costs (interest, rates, insurance, repairs etc..) that are not claimable can still be added to the cost base of the property for CGT purposes, so ultimately you will still get a tax benefit from these costs, just at a later stage (and to a lesser degree, depending what ultimately happens with the CGT discount after the May budget...)