The vendor accepted my offer and soon I will be the owner of a beautyful 8,5 ha piece of Australia.
As holder of a temporary visa I have to seek permission at the Foreign Investment Review Board (http://www.firb.gov.au/content/default.asp) before purchasing a property. It took only 3 days and the answer came by email:

"...Under section 25(1B) of the Foreign Acquisitions and Takeovers Act 1975 – there are no objections to the applicant(s) purchasing the abovementioned property subject to the following legally binding conditions imposed under section 25(1A) of the Foreign Acquisitions and Takeovers Act 1975:

• the applicant(s) must construct a house on the vacant land; and
• ongoing development must commence within 24 months of the date of this email...."

This shouldn't be a problem.