A 20 A load on 2.5 mm² copper cable over 30 m drops 10.81 V, which is 4.7 % of 230 V. That is inside the 5 % limit of AS/NZS 3000:2018 Cl 3.6.2, but close to it.
The case
The circuit is single‑phase, multicore PVC cable, at a power factor of 1. The tool reads the cable's resistance and reactance from AS/NZS 3008.1.2:2017 and works the drop at 18.02 mV/A·m. At that rate the run can reach 31.91 m before it meets the limit.
What changes the answer
A larger cable drops less. 4 mm² over the same 30 m drops 2.93 %, and can run 51.25 m.
The limit covers the whole installation, from the point of supply to the load. A drop in the mains or a submain uses part of the 5 %, so a final subcircuit often has less to spend.
See also: AS/NZS 3000 Clause 3.6.2: voltage drop limit and The 5 % voltage drop limit from supply to load.