The SSAA Service Rifle handbook says it has to be "either an original or a faithfully reproduced model, with no enhancements compared to the standard pattern rifle" - which, as I understand it, means rebarrelled rifles are fine as long as it's the same barrel length as the original.
For what it's worth, in my experience no-one is that worried about it at club or even state level; although each club is different so what's "Yeah, that's fine, looks like an old military rifle to me" at one club might not be considered OK at another.