Four All Blacks in the starting line-up for tonight's clash against the Wallabies - including captain Richie McCaw - are officially obese and the rest are overweight, according to their Body Mass Index reading.

The formula was used in an OECD report released this week which puts New Zealand as the third fattest nation in the world, after the United States and Mexico.

The Herald calculated the BMI reading - a combination of height versus weight - of the current members of our top rugby team ahead of tonight's opening Tri-Nations game in Auckland.

According to the New Zealand Heart Foundation website, people of European descent with a BMI reading between 25 and 30 are overweight while those with a higher reading are obese.

And Maori and Pacific Islanders with a reading between 26 and 32 are overweight and those with a higher reading are obese.

Every All Black had a reading of higher than 26.

The three starting front-row forwards - Neemia Tialata, Andrew Hore and Tony Woodcock - were all considered obese.

The heaviest, prop Tialata, had the highest BMI reading. His 187cm height combined with his 127kg weight gave him a 36.31 BMI.

And All Black captain, flanker Richie McCaw, was also considered obese with a reading of 30.1.

Two others on the sideline tonight - Keven Mealamu and Owen Franks - were also obese while the other five in the wings were overweight.

The lightest member of the team, fullback Cory Jane who weighs 88kg, had the lowest BMI at 26.27 but was still considered overweight. The Wallabies fared worse with five of the starting 15 considered obese and five on the sideline also in the obese category.

However they had the only two players of the two teams who were considered a healthy weight. Twenty-two-year-old Lachie Turner, playing fullback opposite Jane, had a BMI of 24.63 and the youngest and lightest member of the team, 19-year-old first five-eighths James O'Connor who is starting on the sideline tonight, had a BMI of 24.69.

But Dr Robyn Toomath, of the Fight the Obesity Epidemic, said BMI should not be used to measure if an athlete was obese.

"Footballers are the classic example for who the body mass index doesn't work.

"It would be a great shame if people tried to deny the existence of an obesity epidemic by focusing on these quirky little exceptions to the rule.

"Unfortunately, it is true that if all of us had physiques like the elite sportspeople, then I would be the first to say that we don't have a problem irrespective of what the body mass index shows but certainly that's not the case."

Dr Toomath said the BMI was usually used to determine whether a person was likely to get Type 2 diabetes but a better measurement would be to take a person's weight circumference. She said she would be shocked if any of the All Blacks had a circumference above 100cm, which was considered obese.

Maree Burns, co-ordinator of the Auckland-based Eating Difficulties Education Network, said BMI didn't take into account several things such as a person's fat to muscle ratio.

"It is possible that New Zealanders have been getting bigger but I don't know that the BMI measure itself necessarily equates to the health of the nation. It doesn't say anything about anybody's physiological makeup. In the case of the All Blacks, that point's made really well."