On a technicality, parliamentary parties don’t pick candidates.
The local branch (electorate) of each political party holds pre-selections in every seat. If the incumbent member of parliament is standing again,they typically win, but they are often challenged and sometimes lose.
e.g. Gary Humphries recently lost ACT Liberal Senate preselection
At the preselection meetings any prospective candidates stands up before the local branch preselection committee and gives their “pick-me” stump speech (with plenty of pre-meeting lobbying/shenanigans).
There is often a stoush between the local branch and the state/federal branch if the Feds try to parachute in an “eminent” candidate in over the locals preferences.
e.g. Gillard faces criticism over Peris Senate preselection
So the political party apparatus determines who will be the candidate.
If they win, the candidate becomes one of the parliamentary political party. The relationship between parliamentary party and the political party is often not fully functional.
In the LIBs the parliamentary party has primacy on what happens in the parliament. If the “rank & file” Liberal party branches want, say Australia to sell off some publically owned institution and the LIB MPs make up their own collective mind. The rationale for this is the MP is responsible for the interests of their electorate as a whole, not the interests of the tiny proportion who are actually party members.
The LABs broadly takes the opposite view, though the LAB leaders and MPs have been winning greater control over their affairs since the LABs started winning elections. To illustrate the doctrine at it’s peak was the 1963 photo of Labor leader Caldwell and deputy Whitlam waiting outside party headquarters while 36 “faceless men” determined what policy they would take on US military bases.
