Hi
1) You should file a I.A under Order 6 Rule 17 CPC seeking to amend the main prayer in RCR Suit and once the I.A is allowed, you will make amendments in your RCR Petition by incorporating the new prayer to declare Australian Divorce decree as Null and Void.
2) Once the I.A is allowed by the court, you should amend the main prayer in RCR petition under Order 6 Rule 17 CPC seeking to declare the divorce decree in australia obtained on the grounds of "Irretrievable Breakdown of Marriage" or "No Fault Divorce",
3) It should be borne in mind that Y.Narasimha Rao judgment applies only in cases where the wife was NOT
a) A party to Divorce Proceedings in Australia OR
b) The Wife was Illiterate and was not aware of legal procedure adopted in grant of overseas decree
Hope this information is useful.