If there is no lease it wil be treated as vacant possession and you will pay GST.
For referance if a comm property is leased it will have a going concern value e.g. Value if sold with current tennent. If it is vacant it will generally be sold on vacant possession value. Valuers will often include both values in a report. Many banks only lend against vacant possession value (unless a very strong deal).
If your property is not purchased as a going concern e.g. Vacant or to be owner occupied – you pay GST. Often major banks will lend the GST component on a short term overdraft unitl you lodge BAS and get the GST back (lent over and above normal LVR).
If your not registerred for GST make sure you buy going concern – alternitivly register for GST, lodge one BAS, get your refund and deregister from GST.
To the best of my knowledge if their is no lease – you will be treated as vacant possession – I could be wrong…