No matter how deep the criticism cut - and there was no end of acid being poured over him in Parliament yesterday - it did not cut deep enough to penetrate Nick Smith's thick political hide.

The intense and prolonged barrage directed at the Climate Change Minister by the Opposition, with some help from Act, National's supposed support partner, would have left many a Cabinet minister feeling somewhat queasy.

Not Smith. He can be headstrong. He can be unpredictable. He can sometimes be the veritable bull in the china shop. He is prone to saying the wrong thing at the wrong time. His cockiness can land him in trouble.

But yesterday the disciplined Smith was positively relishing the challenge of answering the critics of the Government's emissions trading scheme and, more particularly, the controversial last-minute deal which has the Maori Party backing the enacting legislation.

When he is in that kind of mood, Smith's self-confidence, lengthy parliamentary experience, power of concentration and grasp of detail make him an extremely difficult minister for an Opposition to score points off.

He can also be a particularly slippery customer. He repeatedly fudged yes-or-no-answer-type questions posed by the Greens' Russel Norman and Act's John Boscawen, the former asking Smith whether greenhouse gas emissions would increase or decrease under National's scheme, while the latter wanted to know if it was true that the special Treaty clause to be inserted in the legislation had been agreed on just 18 minutes before National's deal with the Maori Party was announced on Monday.

Smith was more intent on staying on-message and repeating National's line that regarding the conflict between cutting emissions and growing the economy, the Government had "got the balance about right".

Smith was referring to the adage that if you are being attacked from both left and right, then you have probably staked out the correct position.

But there is a difference between annoying everyone a little bit and angering everyone hugely. When the tide of opinion is vehement on both sides, what looks like safe ground can suddenly seem less than solid. The measure of that erosion were remarks made by Richard Long, a former National Party chief of staff. The politically astute one-time press galley journalist had described National's emissions trading legislation as a "dog" which, resulting in part from a race-based deal and lacking across-the-board backing, was akin to Labour's death-rattle, the Electoral Finance Act.

However, Opposition references to this damning critique left Smith unmoved. He mustered his other lines of defence - "it's in the manifesto so we're obligated to do it" and the "Australians are doing the same as we're doing".

He even took time out to congratulate himself on being the first Climate Change Minister to have introduced a price for carbon after 15 years of failure by predecessors in the portfolio.

Smith did have a brief queasy moment. But it was of his own making. He quoted a legal opinion. Charles Chauvel, Labour's climate change spokesman, demanded, as is his right, that Smith table the official document from which he was quoting.

Smith suddenly realised other contents of the document - written for ministers in the previous Labour Government - could be prejudicial to the Crown's interests.

His plea that he not have to table the paper was not helped by Labour happily giving its permission for him to do so.

Smith's afternoon was turning to custard. Then Speaker Lockwood Smith found a ruling by a predecessor which stipulated - much to the other Smith's relief - that only the piece of paper from which the minister was quoting had to be tabled, rather than the document from which the quote originated.