Thereās no established āDeputy PMā position. Sometimes it exists, sometimes it doesnāt, just like every Cabinet position is essentially made up arbitrarily to fit a real or perceived need. Some are self evidently necessary and so basically always exist, others come and go, merge and split with the government of the day.
The constitutional convention for transfer of the role and powers of PM on resignation is the leader of the party currently commanding confidence in the House of Commons, so thatās what happened. In order to avoid breaking that conventional system, weāll see an election shortly to allow that confidence to be tested and to satisfy the electorate that the PM, as is not required but nonetheless is customary, sits in Parliament.
Hypothetically, a party could establish in their party constitution that they shall have a deputy leader who, when forming government, shall be appointed to a cabinet position of deputy PM, and also that should the leader resign while in government, leadership automatically passes to the deputy leader. That would automatically create a passage from PM to Deputy on resignation while in governmentā¦ But, thereās no rule that you have to be an MP to be in cabinet. Cabinet could, and at times in the past has, included Senators. Or someone not in the legislature at all, as is currently the case,
Aging the existing constitutional conventions would require formally codifying the office of Prime Minister into our written constitution. That would be opening a major can of worms.
Weāll have an election very shortly that will settle this to the satisfaction of voters.