The unions aren't the basis, Labour is the workers party, National is the businesses party, so both sides have their two key sectors of the population. If Labour moved away from being the workers party, then it would be a party the doesn't support workers or businesses, now that would be weird. The unions are just more obvious than the relationship that National has with businesses, other than that its just the same
National is not affiliated to any group in the same manner Labour is TIED to the unions. National has NO group with whom they must comply. You are making some dangerous assumptions about something you know little about.
National voters are from all sectors of the population. I could say that National voters are the educated voters, but that would be an assumption like I see in this thread.
All financial donations to National are voluntary and business's actually donate to all parties including Labour. However, with Labour a portion of the Unions subscription gets paid directly to the Labour party as of right. Quite wrong imo.
Unless within 3 months of an election, the Union must also agree to the appointment of the Labour Party Leader. Also quite wrong imo.