A new study suggests 750,000 Kiwis have been taking vitamins and diet supplements habitually for years -- but nutrition experts warn many are probably wasting their time and money.

The Southern Cross Healthcare Group survey found 35 per cent of Kiwis routinely used supplements, and women were far more likely than men to take the products.

The last New Zealand Adult Nutrition Survey, completed in 2009, found only 30.7 per cent of New Zealanders were regular supplement users.

Some vitamin products do already point out that supplements should not replace a balanced diet. But the new survey of 1650 people sparked fresh calls from nutrition experts about balanced diets.

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The University of Otago's Professor Jim Mann said many people who needed supplements couldn't afford them, and those who did take vitamins often did so needlessly.

He said supplements were often "hugely" overrated.

"It's basically the rich folks, who are worried about their health and take supplements. And perhaps the poor folk who are not so worried about their health don't take supplements, because they can't afford it."

The Southern Cross survey suggested some 1.56m New Zealanders regularly took the products. About 750,000 had done so for at least five years.

Prof Mann said vitamins were often of "questionable benefit other than to people that are advertising them, or people that are making them".

But he said folic acid could be beneficial for women trying to get pregnant. And people with iron and iodine deficiencies also had specific dietary needs supplements could address, he said.

However, Prof Mann said this did not undermine the role a balanced diet, genetics, and a healthy lifestyle played.

"You can show that some of the vitamin deficiencies do look as if they might increase the risk of certain diseases. But when you actually give these as vitamin supplements, they don't really do much good ... what you really need to do is have the blend of those vitamins and minerals that exist in food, rather than as supplements."

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Supplements were often good for people with restricted diets such as veganism, New Zealand Nutrition Foundation dietician Sarah Hanrahan said.

But she said supplement overuse could have toxic effects.

"For example, fat soluble vitamins are stored in the body for months and can reach toxic levels and too much iron can increase the risk of serious illness."

Ms Hanrahan urged people to stay away from multiple-supplement cocktails.

"You need to be particularly careful if you take several supplements, as the total amounts may add up to high doses," she said.

Fish oil or Omega 3, multivitamins and Vitamin C were the most popular vitamin choices here, the Southern Cross survey found.

The survey, carried out in June, showed different towns had different tastes and habits.

Of those who did take supplements, Dunedinites were among the tightest when it came to forking out, with 23 per cent of that city's supplement fans spending $5 or less a month on the products.

In Auckland, Christchurch and Tauranga the average monthly spend was $23. In Dunedin, the average spend was just $16.

Cantabrians and Aucklanders were most likely to spend $50 or more each month.

Nationwide, regular supplement-takers spent on average $21 each month on the products.

Some 42 per cent of women claimed to frequently take vitamins or supplements but only 27 per cent of men did so.

Hamiltonians were most likely to be long-term supplement fans, with 56 per cent having done so for more than five years.

Dunedinites were most likely to be novices, with 23 per cent starting within the last six months and only 39 per cent having done so for more than five years.

That survey found fish oils and plant oils were the supplements New Zealanders most often used.

The Therapeutic Products and Medicines Bill, stuck in limbo, would have regulated safety and quality issues around complementary medicines including homoeopathic medicines, vitamins, minerals and dietary and nutritional supplements.