Tasmania did have the most infections per capita until a couple of weeks into Victoria's outvreak, and still has the most deaths per capita largely because its one serious outbreak was in the nation's most stressed health system in ordinary times (sadly, Victoria will almost certainly take over in the next few days).
It also has zero international flights, was the first to impose state border restrictions, and obviously has no substantial land borders to sneak across. The Tas govt even shut Bunnings, one of the congestion points in most places, on the north west coast during the Burnie crisis.
And even the high per capita numbers were relatively low in absolute numbers, making the control easier along with travel restrictions internal to the state.
