I know for a fact that the Australian cars were built to a different specification to the US ones.
They were faster, a major reason why the 260Z lasted from 74 to 78 here but only one year in the us untill the 280z (a car never sold in aus) took its place.
Glass' dealer guide says that the 260Z had 121kW of power, 161hp, the 240 had 151hp. Hence maybe your manual is not too far off.
seeing the main reason the 260 sucked in the us was the carbies, maybe the aus one had better carbies.
either way, the 240 carbies on a 260 give better performance, and alot of 260 have these in anyway
then there was the axtra mass of the 260 which would reduce performace.
One thing is for sure and thats that the 260z Aus was better than the us one, just by how much...i would say quite a bit, maybe not enought to beat a 240 but close i would think
Aust. Classic Car had an article on these two cars in their december 96 issue, except its not on their wesite anymore if you can find it it should shed some light on the issue.
On paper it would seem that the Australian 240 and 260 would be quite similar in performance, but paper is not bitchumen. Maybe the best way is to find a stock 240 and 260 and have a drag