An Ombudsman/ACCC reference number can make a big difference to their attitude. I was having some issues and got to this point, the moment i gave him a case number I got everything i wanted. The beauty was they still get investigated by ACCC =)
You do need to make reasonable effort to find a solution, so i would suggest trying to identify the faulty ones (likely all 6!) and let him replace them all, then when they are still faulty you have full right to demand refund. The law state they can offer 'Repair, Replacement or Refund' but you don't get to chose which, they do. So you do need to go through the long painful process.
Phone the Ombudsman/ACCC and have a chat, they will advise what you should do.