A study from the Murdoch Children's Research Institute has found at least 14 cases over nine months where people had a severe allergic reaction, most likely due to contaminated packaged foods.

It has prompted calls for changes to food labelling, with doctors saying it is impossible for consumers to tell what foods have undergone screening for allergens and are likely to be safe, and those which could pose a threat.

It is presently mandatory for common allergens (such as peanuts, tree nuts, milk, eggs, sesame and fish) to be declared on food labels as ingredients.

However only some food manufacturers disclose if there is a chance their products contain traces of allergens - known as precautionary allergen labelling.

This means in cases where there is no warning, consumers are not able to tell if it is because the product has been checked and found to be safe, or if it is because a voluntary warning has not been included.

Senior author of the study, Professor Katie Allen, said some customers also believed manufacturers used the warnings for legal reasons, rather than a real risk they may contain trace allergens - adding to the confusion and leading some to ignore the labels altogether.

"When you speak to manufacturers they say ???look, if in doubt put a label on', because they want to keep people safe," she said.

Professor Allen said researchers began looking into the anaphylaxis cases with the hypothesis that reactions caused by packaged foods were relatively uncommon.

But the survey of 198 health practitioners who specialise in allergies threw up unexpected results.

It found 14 cases of anaphylaxis linked to packaged foods, including eight cases where there was no precautionary labelling, or the precautionary labelling did not name the trigger food.

"We were quite surprised," Professor Allen said.

"Our study showed that anaphylaxis to undeclared allergens is not rare and it did not appear to depend on whether the product was labelled with precautionary advice."

Food allergen contamination has been on the radar of authorities, especially after Melbourne boy Ronak Warty died in 2013 after consuming a coconut drink later discovered to contain undeclared dairy milk.

Emi Habgood, a Year 10 student at Princes Hill Secondary College in Melbourne, said it could be stressful managing her allergies to wheat, eggs and peanuts.

The 15-year-old said while she was more likely to have a reaction to food prepared by cafes and restaurants, she did recently have a reaction to a rice biscuit that she had previously eaten with no problem.

"We kind of assumed there was a cross contamination with other things," Ms Habgood said.

"We went over the ingredients and there was nothing there."

Undeclared allergens were the leading reason behind food recalls in Australia and New Zealand in the last year, responsible for 26 recalls. And just this week, a batch of Leggo's basil pesto was recalled because the label failed to declare cashew nuts.

Food Standards Australia New Zealand said some of the problems were caused by reformulation of recipes, packaging errors or accidental cross contamination.

A voluntary program called VITAL backed by the food industry has introduced consistent "may be present" warnings for cases where cross contact of food allergens can occur.

But the problem remains that consumers are unable to tell which products have been screened by the program.

Dr Lara Ford, clinical lead of the New South Wales anaphylaxis education program, said "there is nothing on the packaging to say the food has gone through the assessment process" and called for a mandatory system of "well-regulated" precautionary labelling.

"In the meantime, an indication on packaging that shows a product had gone through the VITAL assessment process would be useful."

Tom Lewis, chief executive of the Allergen Bureau which runs VITAL, said the bureau was aware of this issue and working on a certification system, where VITAL-accredited products could be identified.

But a spokeswoman from Food Standards Australia New Zealand said there were no plans to make precautionary allergen labelling mandatory.