National is calling on Helen Clark to investigate alleged donations by Owen Glenn to New Zealand First (which, if they happened, were perfectly legal). Consider what that means: National is advocating for the leader of a political party to be able to delve into the internal finances of another political party.
That would be a charter for corruption; a grossly undemocratic power for one political power to wield over others. There’s no way if John Key were PM that he would claim the power to investigate another party’s finances in similar circumstances.
National knows that. They don’t really want Clark to investigate NZF’s finances. It’s just that this is the political scandal of the day. They can’t attack NZF, whom they might need in a post-election deal to govern, but they’re always keen to find a way to smear Labour. So, they’ve generated this ridiculous hit and run attack.