The minimum copper conductor for a lighting circuit is 1 mm². AS/NZS 3000:2018 Table 3.3 gives that size for every circuit other than socket‑outlets, and a lighting circuit is one of them.
A floor, not a design
The minimum size does not account for the load or the run. A lighting circuit over a long run reaches the voltage drop limit first, and the cable size calculator then selects a larger size.
A socket‑outlet on the same circuit changes the row. Table 3.3 then sets the socket‑outlet minimum instead.
See also: What size cable for a lighting circuit in NZ? and Minimum cable size for power points.