So far, so farcical. The Prime Minister's defence of John Banks looks more ridiculous by the day.

If ridicule is the price of preserving the Government's thin majority, however, John Key will continue to wear it - even if less than willingly.

Key is fairly confident of the direction in which the Banks donations scandal is heading: nowhere. He is gambling on his fig-leaf defence of his refusal to boot Banks from his ministry outlasting the capacity of Opposition parties to keep the story alive.

The risk is that even more damaging material will seep out. But Key's test of whether Banks stays or is sacked has always been stacked in Banks' favour.

Advertisement

That test is whether Key can rely on the MP's word. Banks has given his word that he complied with the law, and the police say there is insufficient evidence to prosecute.

This saga has always been about power - more exactly, the retention of power. National is willing to burn some political capital to continue to get backing from Banks plus United Future's Peter Dunne for right-leaning measures, while counting on the Maori Party for things more centrist.

Yesterday's sitting of Parliament was the Opposition's first opportunity to put Key's defence under real pressure since the Prime Minister's return from Russia and Japan last week.

Opposition MPs used logic, sarcasm and even satire to try to break down Key's rationale for not disciplining Banks. They did not succeed.

At one point, however, the Greens' Metiria Turei threw Bill English's "Planet Labour" insult back at Key, questioning whether Banks' behaviour would be acceptable on "Planet Key".

Key professed not to know much about Planet Key, but expected it would be called Nirvana and be a lovely place to live where "golf courses would be plentiful".

He must have been tempted to add that Planet Key would be a place where prime ministers would not have to defend the indefensible.