Using the NPL only (as the crow flies, centre circle to centre circle, and not counting shared ground derbys):
ACT: Queanbeyan City and Monaro Panthers (Queanbeyan / Riverside Derby) @ 350m
NSW: Marconi and Sydney United @ 1470m
VIC: Dandenong City and Dandenong Thunder @ 5320m
Others will probably have to share what the biggest 'derby' matches are in each NPL as it stands, as I'd only be speculating - but give me some time to get the competitions added to
https://map11.net/ and I'll work out at least the closest non-shared venues in each NPL for 2026
SA: Croydon and West Adelaide @ 1934m
WA: Dianella White Eagles and Bayswater City @ 4500m
QLD: Eastern Suburbs and Olympic FC @ 5036m
NNSW: Adamstown and Newcastle Olympic @ 986m - although Broadmeadow Magic and Kahibah also 1017m apart
TAS: Launceston City and Launceston United @ 6542m