I am expanding my question here, as I am unable to add to my original post.
My understanding is a workshop pit needs mechanical ventilation under AS/NZS 60079 ZA.4.1, along with lighting and electrical equipment being "intrinsically safe". If this is in place no other warning systems are needed. ie an adequate fan ventilation system and correct lighting/electrical equipment is enough to ensure the ventilation of the pit is safe for workers to access.

I would suggest you consider the broader terms of the legislation.

(1) A duty imposed on a person by or under this Act requires the person—
(a) to eliminate risks to health and safety, so far as is reasonably practicable; and

(b) if it is not reasonably practicable to eliminate risks to health and safety, to minimise those risks so far as is reasonably practicable.

The standard (I have only seen the free bits) considers explosive atmospheres. There are going to be other risks that you need to manage. A occupational hygienist would be a good person to start with looking at what the hazards are. They can give you direction on how to manage the hazards.

Some examples of possible hazards are carbon monoxide, formaldehyde, black carbon/diesel particulate, noise, lack of oxygen.

The air quality in workshops with doors at each end may not need additional ventilation. Keeping the air still and warm is often a bigger issue. You can buy or hire a dosemeter that your technicians wear for a day to measure their exposure.

you highlight a good point that the situation can easily change - workshop doors open during summer to get a good breeze through, compared to shut up and heaters on to warm the place up in winter - completely different risk profiles. Remember that if you are renting dosimeters / personal gas monitors, that you should also record the situation/environment during the testing for reference.

Definitely.