Hi mate,
I assume you mean Lake Kurwongbah, we have caught one feral goldfish on an electrofish sample earlier this year. There are a tonne of small Tilapia in there which are illegal to keep, they can have a goldish colour but normally also have vertical black stripes as well.
The group I'm involved with, PRFMA, recently realeased in Kurwongbah some 70mm Golden Perch which are a pale golden colour. By the letter of the law taking these fish would be against regs due to being undersize, in terms of the morality of taking them you can make your own decision. Due to it being a bad season breeding wise for Golden Perch we were unable to obtain anywhere near the numbers we wanted. So for a first release of this species it was disappointing. Hopefully the coming season will be better.
There are bony bream present, also firetail, flathead and carp gudgeons, some spangled perch and one of my favourites eel tail catfish.
There have also been Bass released which hopefully should be at least 10cm by now. Also the start of a breeding population of gar fish is in the lake now, only 500 of them but if they get left alone there will be plenty of them soon enough.
We are probably still 2 years away from the Lake starting to be a fun place to fish.
If you can, get a photo of the fish in question and post it on here for an I.D.