Interesting discussion all...
While I believe that there are some CAAs that permit a ratio of 1:50 passengers, the basic CASA proposal is based on the number of configured seats (1 cc per 50 seats installed).
There really is no "international standard" to speak of as ICAO hasn't specified one. Chapter 12 of Annex 6 to the Convention on International Civil Aviation relates to the requirements for Cabin Crew. Section 12.1 addresses the Standard for the assignment of emergency duties as follows:
"An operator shall establish, to the satisfaction of the State of the Operator, the minimum number of cabin crew required for each type of aeroplane, based on seating capacity or the number of passengers carried, in order to effect a safe and expeditious evacuation of the aeroplane, and the necessary functions to be performed in an emergency or a situation requiring emergency evacuation. The operator shall assign these functions for each type of aeroplane."
This isn't the first time that a change to the Australian ratio has been proposed. However, this time it is likely to be adopted as CASA has already been permitting the practice through special authorizations as noted earlier. If the Australian ratio does change, it would leave Canada as the only large CAA with a ratio based on the number of passengers carried rather than the number of configured passenger seats. A similar proposal here that would have permitted the option of operating to the existing 1:40 passenger ratio or the proposed 1:50 seat ratio was stopped by the Transport Minister in 2006 following a public campaign opposing the change. An archived version of the website opposing the change can be seen here:
Airline Passenger Safety - Feb 02, 2006
Airline Passenger Safety - Sep 19, 2007
It will be interesting to see the outcome of the CASA proposal.
Since ~1967, when the requirement for a manufacturer to demonstrate the evacuation capablity of their aircraft was first introduced to the certifcation requirements, the ratio used has been 1 cc per 50 seats. This was due to the fact that it was the most critical ratio being permitted at the time in the various operating rules. So, even here in Canada where the ratio has been 1 cc per 40 passengers since ~1968, the Canadian manufacturers have used a ratio of 1:50 in order to market their aircraft worldwide. The Canadian operating rule will supercede the certification basis by requiring a lower ratio for Canadian operators.
For a sense of which aircraft were certified by the manufacturer via actual demonstration and which were completed via analysis, you can review the data at the following link. It is an FAA publication but includes the manufacturer information from a wide range of manufacturers:
Maximum Passenger Seating Capacity for Airplanes Used in 14 CFR Part 121 or 125 Operations