What was the Statute of Westminster?
In 1931, the British parliament passed the Statute of Westminster, a law that gave formal recognition to all countries with Dominion status full independence, stating that they were bound by no laws but their own. This gave the dominions complete independence, but in some cases complete independence in some areas were not achieved until later. ex: The Supreme court of Canada was the Privy Council in London until 1949. The power to amend the Canadian constitution remained in the hands of the British until 1982 because the provincial and federal governments couldn't agree upon the subject. Answer In order to make this full independence a complete reality, the Dominions - Canada, Australia, New Zealand and South Africa - had to pass their own legislation on constitutional matters in order to ensure that there were no legal gaps.
The delay referred to above in the case of Canada was exceptionally long.