I agree that the two species Litoria leseuerii and L.wilcoxii are very similar and not easy to tell apart from a photo. In fact, they were only fairly recently taxonomically separated.
The best way that I know of to distinguish between the two species is the groin patterns, which is black and blue in L. leseuerii and black and yellow in L.wilcoxii . Unfortunately we can't see the groin in the pic provided, so unless more pics or information from the OP are forthcoming, then we can't use this distinguishing feature.
Klaery, like you I used distribution to go with L. leseuerii, as the range limit for both species is usually given as the Hawkesbury River. However, one source mentions the Nepean River, so there may not be much in it. I'll check for any reliable records below Sydney.
Does anyone happen to have a copy of the paper separating the two species that they wouldn't mind sending me?
- - - Updated - - -
I've since found reliable records of sightings and calls from below the Hawkesbury River. One observation is from the Georges River near Long Point and several down at Dharawal SCA on Stokes Creek. So the range limit or cut off point of the Hawkesbury River given in some references for these species is apparently incorrect. Therefore this specimen could indeed be either Litoria leseuerii or L.wilcoxii.