Well, except if the law goes and makes it a problem. Here's Cullen Law's discussion of obesity and employment.
New Zealand law imposes similar [to the previously discussed Australian case - hit the link] obligations on employers under the good faith requirements of the Employment Relations Act which requires employers to be “active and constructive” and “responsive and communicative”. Additionally, employers must consult employees before a decision likely to have an adverse effect on the continuation of their employment is made.Doesn't seem a big issue thus far. But if it becomes difficult to dismiss people for not being able to do their job where obesity is to blame, then the combination of a bad law and obesity would make for externalisation of cost. And it'll be tallied as a cost of obesity rather than as a cost of stupid laws.
In 2014 the New Zealand Employment Court case of Dunn v Waitemata District Health Board discussed the extent of employers’ obligations when dealing with an employee who is medically unfit for work. The Court stated that an employer is not required to keep a job open indefinitely where an employee is suffering from a prolonged illness. Much depends on the circumstances, including the employer's needs and what can and cannot reasonably be accommodated, and the anticipated timeframe for any return. A fair process must be followed. In the Dunn case, the dismissal was also held to be justifiable.
Alternatively it is possible that in a case similar to Parahi’s, that the law of frustration would apply. Frustration occurs when there is an unforeseen change of circumstances, through no fault of the parties, which means that the performance of an agreement will be either impossible or something radically different from that which was contracted for. Such cases are not considered dismissals but rather a termination by operation of law. The usual circumstances of frustration of employment relationships include sickness or injury.
Although the chances of being fired for obesity are very minimal and limited only to where it would impinge directly on your ability to fulfil your job, it is an interesting problem that has only recently been brought to the fore. Obesity has historically been a non-issue but as the 2015 New Zealand Health Survey found that 31% of adults in our country are obese, it is bound to be a developing area of employment law. I am sure we all, as New Zealanders, will watch with interest whilst trying to avoid being in the 31% of people at risk.