Burgers might have given fast food its unhealthy reputation, but the most kilojoule heavy meal among restaurant chains is neither a Whopper nor a Big Mac but an unassuming bowl of nachos.

A bowl of pork nachos with garlic sauce from the Mexican food chain Zambrero contains 5005 kilojoules - more than double the recommended kilojoule requirement for an average adult meal.

This was enough to eclipse the kilojoule count of an Ultimate Double Whopper from Hungry Jack's (4773 kilojoules), a Double Quarter Pounder from McDonald's (3570 kilojoules) or a Bacon and Cheese Rippa from Red Rooster (3580 kilojoules).

The average Australian adult consumes about 8700 kilojoules a day, and the recommended amount is no more than 2000 kilojoules per meal.

The George Institute for Global Health analysed 229 small meals and snacks from 25 fast food chains, based on the material they provided to the NSW government's menu board labelling program.

Researchers then compared the kilojoule count with the amount of exercise that would be required to burn it off, which in the case of Zambrero's pork nachos equated to running for an hour and 48 minutes, walking for four hours and eight minutes or cycling for two hours and 25 minutes.

Food from Mexican fast food restaurants was among the least healthy, with the three most energy rich meals from the Guzman y Gomez and Mad Mex franchises each requiring a run of more than 90 minutes to burn off the kilojoules.

The Mexican wave of fast food restaurants rose about 10 years ago, and some are among the fastest growing franchises in Australia.

Zambrero opened in Canberra in 2005 and appeared in the BRW Fast Franchise list for four consecutive years from 2011, since which time it has expanded into New Zealand and Bangkok.

It prides itself on its socially responsible values, and delivers one meal to a person living in poverty for every bowl of nachos that is sold.

George Institute researcher Kristina Peterson said a quarter of all the meals examined exceeded 3000 kilojoules, which is a third of the average kilojoule intake of an Australian adult.

"There were some quite alarming results," Dr Peterson said.

"A lot of the Mexican meals didn't do particularly well, which might be surprising to people. A lot of salads are laden with sauces and dressings which increase the amount of kilojoules."

A small ancient grain peanut orange salad from Sumo Salad would take a run of more than an hour to burn off, while a coconut mango salad from Jamaica Blue would require a run of an hour and 18 minutes.

A spokeswoman for Zambrero said that on the organisation's calculation the pork nachos only amounted to 4198 kilojoules, which was lower than the same dish offered by its competitors Guzman y Gomez and Salsa's.

"While this dish has the highest calorie count on our menu, Zambrero is committed to providing healthy, lower kilojoule options for our health conscious customers," the spokeswoman said.

"We have seen very positive feedback and an increasing uptake across the board for our healthier options such as our IQ and Powerbowl Range. At our newly opened restaurant in Coogee, these dishes make up more than a third of our total sales and are continuing to grow in popularity."

A spokesman for Guzman y Gomez said the menu options ranged from soft tacos with only 1510 kilojoules to chicken enchiladas with 4730 kilojoules.

"At GYG we believe in being transparent about our menu and giving customers choice," the spokesman said.

"We actively promote healthy suggestions in our Taquerias and believe the healthiest food is fresh real food."