The tax is levied on one date - so if the property was rented out or available for rent at this date then it would probably be deductible for the whole year - check with your tax advisor.
In NSW it is assessable as of midnight on 31 Dec each year. VIC is probably similar. If it was rented out on this date then you would pay the full year's land tax - and should be able to claim this as a deduction.
