Trengove was closing argument before the Constitutional Court in Johannesburg on whether the proposed final constitution could be certified.
He said the Act was not part of the constitution as some people thought.
There were certain permissable exceptions to the general rules of any constitution, Trengove said.
He explained that the Act fell within the ambit of a specific and limited exception, and the legislature was not left any discretion to amend the Act, except by the special procedures required for constitutional amendments.
Constitutional immunity did not extend to any amendment made to the Act, he added.
Trengove said the Act had been given this special status because the Constitutional Assembly had been anxious to ensure the Act was not rendered unconstitutional when the final South African constitution was certified.
"It is an insurance policy," he said.
Trengove said the Labour Relations Act enjoyed the same status.