What is John Key hiding?

Andrew Little has laid down the gauntlet on transparency and tax cheating, announcing that he will release his tax returns. As for John Key, not so much:

But Key said he has no intention of releasing his records. He was "100 per cent confident" his tax affairs are beyond reproach.

Yeah, nah. Its not a question of whether he's confident - but whether we are. This just comes across as more shiftiness. To point out the obvious, the Prime Minister used to be a banker; what is he hiding?