Parties with significant community support will win seats in every part of the country and risk losing seats if they take any area for granted. Voters have a wide selection of candidates and several local members to whom they can take both constituency and political matters. It's worth fighting for a system where voters play a key role on election day. Usually you have a fairly good idea of the overall outcome on election night. In Tasmania and the ACT, actual counting takes from two to four days once the quota for election has been calculated and transfers of votes begin as candidates are declared elected or excluded.
Parties have to convince voters their policies and candidates are superior.
They can't seize unchecked power if they're seen as just the best of a fairly bad lot. By the same token, they don't become a infighting irrelevant rump if voters decide to punish them. Also, between 1934 and 1969 Tasmania produced the longest unbroken run in government for any party in Australia. You don't need by-elections. Casual vacancies can be filled by re-examining the votes for the outgoing member.