When and how did mosquitoes reach Australia?
It's east of the Wallace line, so 'they got there from Asia' is not an obviously unproblematic conclusion.
In general, when and how mosquitoes reached remote Pacific islands is a nontrivial question. For example, Hawaii was free of them until historically recent time when they stowed away on a ship.
Study finds introduced mosquito species active all year round suggests one particular mosquito species only reached Australia during World War II, but also says
Australia has plenty of 'home grown' mosquitoes
Which indicates that other species have been there much longer.
Did they reach Australia at the same time as the marsupials, tens of millions of years ago when the continent was closer to Asia?