Welcome to ECN,mate!.
I have lots of experience with Fluorescent Lighting.
Invariably my Faults work, means I fix heaps of Under-verandah fittings hooked up to the Street-Lighting circuit, in town here.
So are we talking a Twin tube fitting?
Usually a Ballast will have a reasonably low resistance, our 230V ones over here, have a resistance of 57 ohms when new.
What is the fitting doing?, no light from one tube?, Tube flashing?.
Check the starter and the lampholder contacts first.
Then discount the ballast.