gillard too cunning by half, page-5

AFAIK it depends on the Speaker.

If the is the need for a by-election, then the Speaker can decide whether to defer until Sept 14. OR she can decide to hold one before then.

Because there will be another sitting of parliament just before the Sept 14 date it is likely that she would call a by-election before then.

The Govt can stand to lose one seat only, so the chances of an election needing to be called before Sept 14 are pretty slim.