Technically it was the ratification of the Statute of Westminster that formed the formal declaration of independence for the Old Dominions. This is New Zealand’s official interpretation so that dates our independence in 1947 and this interpretation pushes Canada’s independence to be 1931 and Australia in 1942.
A dominion is, strictly speaking, a self-governing dependent territory rather than an independent country.
I’m an historian and a history teacher, and I’ve had to look at this in a fair amount of detail.
Independence, in my view, came before the ratification of the Statute of Westminster. That merely formalised the Dominions exercising an independent foreign policy - they were permitted to do so, prior to ratification however, and Australia did so considerably before 1942 in a few cases.