This is what some people overlook. Those agencies all have legal authority to intercept communications if they get (for Police and sis anyway) a warrant for it.
What is being proposed is that the GCSB can continue to do the actual interception on their behalf as they have the expertise. If the bill fails, it won't mean a single less domestic interception. It will just mean interception infrastructure will be duplicated and exist in multiple agencies, rather than one.
That's not to say there are not some changes that can be made to the bill.