Why no oral exam for phd in Australia?
Background: In Australia, PhD theses are examined by at least two external examiners; they must be more than an arms length away from the student or supervisors; e.g., no previous collaboration or have worked recently at the same university. Critically, they must be internationally recognized experts in the student's subject matter.  The examiner reports are then deliberated upon by a committee with members drawn from across the university; this means the award of a degree is not done at the school or faculty level.

The examiners serve as independent auditors of the quality of an Australian PhD thesis.  They certify/vouch that an Australian thesis is at an international level.  They do this by presenting a report that justifies that a thesis contains 'significant contributions that extend the state-of-the-art'.

To achieve the said claim for an oral exam, Australian universities will need to either hold video conferences or fly examiners in.  The former is an admin nightmare; e.g., different time zones, getting all involved people to attend/agree, etc.  The latter is too expense.    Now imagine doing the former or latter for hundreds of theses every year. 

Having said that, at my university, oral defense is an option that can be requested by an examiner.  This is only for very exceptional cases and rarely granted.