My other question is: what is the accuracy required to legally fly IFR? And where is it documented.
The required accuracy of Australian IFR altimeters is in the AIP:
When two altimeters are required for the category of operation, one of the altimeters must read the nominated elevation to within 60FT. When the remaining altimeter has an error between 60FT and 75FT, flight under the IFR to the first point of landing, where the accuracy of the altimeter can be re-checked, is approved. In the event that the altimeter shows an error in excess of 60FT on the second check, the altimeter must be considered unserviceable for flight under the IFR.