There are couple of things. The department doesn't want you staying more than 12 months in any 18 month period, and you need to be sure that the department doesn't suspect that you are work. Otherwise you could do it the same day. However, I suggest a trip to NZ or Bali. It is always worth staying a few days in these places.
There is recent case of a returning visitor having his phone checked by a customs officer. The officer found evidence that he had been work in his SMS messages. So take care to delete any messages that may indicate that you have been working.