A question re separate earthing cable for lighting

I can't find a definitive answer in the AS/NZ 3000 regs for this:
Scenario:
An older house with twin 1.0mm TPS without earth, run for all the lighting circuits.
Proposed new light fittings at some points have exposed metal so a separate earth is required.
Logically a separate yellow/green earth wire should be run from the switchboard alongside the original TPS to each lighting point. Acoording to AS/NZ 3000, this should be a minimum of 2.5mm! So far so good.
What is not clearly defined is the the insulation requirement for the separate cable. Can it be standard conduit cable with a single insulation layer of PVC, or does it have to be TPS over the coloured PVC insulation?

Heres me thinking minimum earth bonding wire has to be 4mm, not part of the original TPS so treated as separate bonding wire.

If single layer of insulation, is it such a biggie being a 'non' conducting wire if insulation is damaged or compromised is it unsafe having copper conductors exposed being an earth.

Dont answer your question really, but its how you interpret the regs and 'common' sense should prevail.

It's a PEC, not bonding, so only 2.5 required. Single insulated is fine. It doesn't have to follow the twin cables, but it does have to go to a switchboard, not to a power or lighting point earth.

No problem. Your electrician will know what to do anyway.