If you have a list of all the worksheet names, named Sheetlist then you can use this formula
=SUMPRODUCT(SUMIF(INDIRECT("'"&Sheetlist&"'!AZ6:AZ1721"),">0"))/SUMPRODUCT(COUNTIF(INDIRECT("'"&Sheetlist&"'!AZ6:AZ1721"),">0"))
Edit: that will work even if you have negative numbers but, if not, Biff's suggestion will be preferable