Threat of a debt collector

Keeping some details vague in case this gets back to someone and used against us, an old boss is trying to charge a debt of over $10,000 for the use of some equipment for a home project. Other employees and us in the past have used this free of charge, if we had gone through a reputable hire company it would have been $4000 cheaper at least, and the charge was only sent after he left the place of employment on bad terms and basically the boss has got the shits on.

NO contract of agreement was in place, nor was there a written or verbal agreement of any sort that payment would be owed for the use.

They have continued to send repeated invoices for the last few months and have refused to send proof of contract or agreement after our initial dispute of the charges, saying something along the lines of there was ‘an understanding that payment for use would be worked out at a later time’. (Like anyone would agree to a charge they don’t know the cost of!)

Now they are saying this is the final invoice before they involve debt collectors. They have used lawyers before and have not gone down that path this time. (Yet I suppose)
Help, what are our options! Looking for advice.
Location: South Australia