hi there,
i've camped at cataract a bunch of times and it's a nice campground. the park people tell me it rarely fills up (long weekends it does of course) but you shouldn't have a problem finding a campsite there. it's first come first serve and with both campgrounds you have some nice brookie fishing nearby. nothing massive of course but really convenient. and it's a short drive down the trunk road to get the livingstone river which has bigger bulls and cutts.
fishing cataract and etherington should be pretty simple. just bring your dries in 14-18 in different colours of parachute adams, caddis, humpy's etc and maybe a few smaller stimulators. most of the brooks in there are 8" long but there's a few largeer brook trout in there but you might have to hike a little ways away from the campground. if you hike downstream on cataract, there is a nice waterfall and there can be some bigger fish downstream of there. it's a nice and easy hike for you and the GF. i haven't explored too much upstream of the campground.
you could also try the livingstone falls campground which is on the livingstone river. i've only parked there but the fishing was alright there as well.