Prime Minister John Key says he's not "down in the mouth" about recent polls showing a fall in support for National.

He has dismissed suggestions the loss of support for his Government is only because of last week's humiliating backdown on its plan to increase class size, saying voters are also angry because of the rise in tobacco tax.

A 3 News-Reid Research poll released last night showed support for National down four points to 45.8 per cent, while Labour was up 3.8 per cent to 33.2 per cent.

A TVNZ-Colemar Brunton poll released last week also showed support for National down four points, to 47 per cent and Labour up four to 33 per cent.

Both polls were taken after the Government released its controversial education policy but before Education Minister Hekia Parata was forced to ditch it after concern by parents and threats of strike action from teachers.

Key this morning said the shift in the polls wasn't because of one issue.

"I don't actually accept that," he told TVNZ's Breakfast programme.

A quarter of New Zealanders smoked and the Government was putting a big cost on them, he said.

Under changes announced in the May Budget tobacco excise will increase by 10 per cent a year beginning in 2013 for the next four years, in addition to the annual inflation-indexed increase.

It will increase the price of an average pack of 20 cigarettes to more than $20 by 2016.

"There will be some of those people who say I don't like that and I'm going to reflect that in the poll," Key said.

The Government was raising tobacco tax because it "cared about them and we want them to live".

Governments had to "stand up for things they believed in", Key said.

The Government faced a lot of challenges, he said.

"Of course from time to time as you undertake reform, there will be little bit of push back," he told Radio New Zealand.

However, both polls put support for National at about what it had on election night last year.

"I don't feel particularly down in the mouth about it."

Bigger class sizes were always going to be a "slightly difficult argument" to sell to the public.

"My view is that is better to accept sometimes that we could have had months of industrial action and a lot of anxiety from parents or accept they don't feel comfortable with that step so we will find other ways to lift teacher quality.

"Given that is was a $43 million spend out of $10 billion, sometimes it's better to say people don't feel comfortable with that particular route, let's find another route."

- © Fairfax NZ News