DFC,
You’re right - when the light spacing is 60m it is not mentioned in the (latest) ERSA because that is the standard spacing. It is only mentioned when the spacing is more than 60m.
I checked with Kalgoorlie Airport (60m on main, 90m on cross runway - both are non-precision instrument runways, and the latest ERSA has only the 90m spacing on the cross runway mentioned). Kununurra said they have 90m spacing and I see this is mentioned in the latest ERSA. Karratha said 60m spacing and I see this is not mentioned in the latest ERSA. But **** airport (name omitted) "don't know the spacing? 90m I think. I'd have to go and pace it out". Their light spacing is not mentioned in the latest ERSA.
One airport made the comment that the latest aerodrome audit by CASA required non-standard spacings of 90m on non-precision instrument runways to be noted in ERSA. I guess this is a 2005 innovation and is a rolling change and they haven't got to **** airport yet, but within a short space of time the spacing of lights at non-precision instrument runways in Australia should be 60m unless otherwise noted in ERSA.
I wonder how many pilots have wrongly (unsafely) calculated RVR based on light spacings? I'll leave it to one of them to comment. Your thought of assuming 60m in the absence of other information makes sense.