Can the equipment tip if operated poorly on level ground?
Is the access-way always level?
Could staff ever ignore your rules?

If its there, then yes it is mandatory.
recently at a H&S forum, it was discussed that a Worksafe inspector was out and about, he was in a business for a totally different issue but saw forklifts and a small excavator in use, upon more inspection it was found that none of the operators were wearing seat belts, the company then got a improvement notice and was advised all workers had to do retraining on all their mobile machinery.

It is safe to assume the seatbelt is a safety device, chances are the manufacturer spent an enormous amount of money and R&D to install it when the plant/equipment is made. I highly doubt they spend that time and money without a reason (yes I’m sure someone will pipe up and say its to cover them from litigation…but NZ is a no fault state).
It would be an interesting argument saying you decided not to use something the maker of the plant/equipment deemed necessary.
This applies for just about everything, if it was put there by the people/company making it, who am I to say it is not required.

It would be an interesting argument saying you decided not to use something the maker of the plant/equipment deemed necessary. — Robb

And if you are having to say "we didn't think we needed to because [enter reason] " then there is a good chance that something untoward has happened...

Sorry, should have been more specific - the machines that DON'T have one currently, the question I guess is do they legally need to be installed?

I am sure I will be corrected if I am wrong, to my knowledge there is no ‘legal’ requirement to fit seatbelts for fitting sake.
If, however your risk assessment of the work identified operators being thrown out of the cab (and crushed) during a roll over and a known control is wearing seatbelts, then yes you have a ‘legal’ responsibility to fit them, so far as reasonably practicable.
But…if fitting the seatbelts is not reasonably practicable, as in that the cost of installing them is grossly disproportionate to the risk, then again you don’t have a ‘legal’ responsibility to install them.
I am guessing you have other controls in place such as trained and competent operators, operating with the doors closed, roll over protection structures (if fitted), and flat and level ground which is suitable to carry the weight of the machine. Then these combined controls make the risk of roll over low. Therefore with all these assumptions, I would suggest you probably don’t need to retrofit them.

The key is "when fitted with a protective structure". The protective structure is ineffective unless a seatbelt is worn. In the Manufacturers supplied Operators Manual there will be a requirement for a seatbelt to be worn if the machine came with an OPS , if retrofitted the engineering designer will also have stipulated this.

On a somewhat related topic what say the group to a situation where an operator has a medical certificate exempting them from having to wear a seat belt due to impracticality or undesirability? This raised itself for me today and created a predicament or conflict between what is legally and technically allowable but what doesn't quite sit right in terms of risk and 'what if' scenarios.

for their own safety and the companies liability if something should happen to them, As a H&S professional I would not be allowing any undertaking for any job on that machiney or any others for that fact, but rather find them alternative duties if possible.

Hi.
NZ H & S Regs 1995 Reg 19 & 20
Self-propelled mobile mechanical plant
(1) Every employer must, so far as is reasonably practicable, ensure that every
self-propelled mobile mechanical plant to which this regulation applies is fitted
with a roll-over protective structure and a seat belt.

Regulation 20 does not apply to any self-propelled mobile mechanical plant
that has been designed to be used on level ground at all times, if the employer
has, so far as is reasonably practicable, taken steps to ensure that every employee
at every workplace under the control of that employer so uses the self-propelled
mobile mechanical plant.