The purchaser is fully liable to withhold and pay the amount. If withholding does not occur when it should, the ATO will hold the purchaser liable.
For example Ben purchases a property from his friend Tom for $3,000,000. Ben has known Tom for many years and knows that Tom is an Australian resident so before settlement he does not bother to receive a clearance certificate from Tom. Ben at settlement pays Tom the full $3,000,000. The ATO holds Ben responsible to pay the 10% withholding to the ATO. Ben cannot find Tom and now has to pay an additional $300,000 to the ATO for withholding plus interest.