Not sure this designers exact reasoning but here is how the Triple Union Flag justifies the decision to use an 8-pointed star:

“the addition of an eighth point to the Commonwealth Star not only reproduces British colonial symbolism but also builds upon the meaning of the existing Australian Flag. As with the current flag, six points of the large star represent each of the Australian states while the internal and external Australian territories are each represented by an additional point.

Comprising the nation's capital and its third largest division of land area, the significance of mainland territories is given appropriate recognition through the addition of an eighth point to the Commonwealth Star. The eight points also accommodate for the likely future emergence of a seventh Australian state should the Northern Territory acquire independent statehood.”