Australia starts to close down in the days before Christmas, is virtually closed down in the days between Christmas and New Year and only starts to open in the days after New Year. Australia Post and the public service declare themselves extra public holidays, everything closes down early (including Immigration). Buildings are closed or on skeleton staff. Be acutely aware of this and recognise that if applications are not ledged well before 25 December then documents may get lost, misplaced or not acknowledged.
Never send documents to a building address if there is an alternative post office box address at least, in the latter’s case documents are safely stored at the post office box.
Immigration goes into semi-hibernation starting with the days before Christmas and ending in the days after new year. It is literally a dangerous period for anyone to be doing business with Immigration as often the very inexperienced staff are left on duty and quite erratic decision are made.  If possible try and schedule one’s business with Immigration no later than the last business day before 20 December whenever that might be in any particular year.