Thanks for the responses guys and to Gary for offering to get involved (I've PM'd you). I think the consensus seems to be I'm not being too unreasonable but maybe I should have queried why a previous invoice was so low. And on reflection, I suppose if experience was that it was easy to contact TelstraClear and quickly get a query resolved I might have been more inclined to do that, but I would probably maintain that my job is to pay what's asked for on the invoice, no more, no less.
For those not familiar with the TelstraClear domestic bill/invoice, its main feature is is a big black bold strip in the middle of the page with the words "Total Account Balance" along with a $ number. The detachable payment slip (if you choose to use it) has that same key feature. To me, with a mere 30 years' experience of managing to pay personal and business accounts, this is the information I look for and I (rightly or wrongly) assume it is the amount I am being asked to pay, and indeed is the amount I do pay.
Yes there are a couple of smaller print lines above, one saying "Previous Charges", a second saying "Payments Received" and, looking at the old invoice, Payments Received was greater than Previous Charges.
I understand that billing errors can be made; I understand that it would be relatively easy for someone to type an incorrect account number (mine) into an online payment form resulting in an overpayment to my account and subsequent "reduced" bill to me.
My point is, once TelstraClear realized the nature of these two errors, they should have contacted me and explained the situation and advised that they would issue either a separate correcting invoice or a regular monthly bill with the extra amount added. Instead they simply took the view that the account is unpaid and choose to treat me like someone who hasn't paid their bill for three months, whereas in fact I have not failed to pay what I have been asked to pay. They appear unable to grasp that it is this attitude and their apparent failure to see that I still haven't been asked to pay this amount which is causing annoyance. If I ever get the written explanation and some form of request for payment I'll pay the *!@% thing; if it comes with an apology for the inconvenience and subsequent treatment, then I'll pay it with a smile :-D
Got a letter from TelstraClear's "Team Manager - Credit Services" explaining how the billing error happened, including the line "...acknowledge your concerns...and apologise for this."
So almost what I asked for, although the continued reference to an "overdue" amount, incorrect references to invoice numbers and corresponding values, and the patronizing "...happy to allow an additional 14 days..." still suggests they never really understood what I was complaining about. And is it too much to expect a formal letter from a reasonably large organization to be free of spelling and punctuation errors? Oh, and the fact that I *still* haven't actually had a corrected invoice - evidently a process beyond the sophistication of their billing system...
To answer my original question, it appears that expecting courteous, professional communication of accurate information in an appropriate and timely manner should indeed be considered as being an unreasonable customer. Mea maxima culpa