Au0001

Ideally you should not leave Australia between applying and getting approval, this usually can take a few weeks.  They state you must not but sometimes people have to for work and therefore they need to inform the authorities. You are supposed to be inside Australia when the approval is granted.
After approval and before ceremony ( this can be a few months) you may leave if your visa is still valid or you have an RRV.  You must however be available for ceremony and they notify you by post only in majority of areas, so best to let the council know your plans or discuss likely dates before booking a holiday of more than a couple of weeks.

Hey just a quick question. After apply the citizenship online, do we allow to travel outside of Australia ? Do we have to ask DIBP before traveling?