If you're fishing off the beach in Sydney, then you are more than likely catching sand whiting. According to the Fisheries website, sand whiting are also known as summer whiting, silver whiting and blue-nose whiting. For sand whiting the legal size is 27cm and the bag limit is 20.
sand whiting
The only other whiting you're likely to catch is Eastern School Whiting, also known as red spot whiting. However, they are usually caught offshore in deeper water. They have reddish brown stripes along their flanks. They have no size limit, but the bag limit is 20.
Eastern School Whiting (red spot whiting)
Cheers
Peter