Goby?

It is a goby because if the fins on its back, but which one?

Did the shop not say which species it was? They should have a list somewhere of what they had delivered from the wholesaler.

This seems to be one of the sleeper gobies, generally known as gudgeons in Australia, (Eleotridae) rather than a true goby (Gobiidae). It's probably one of the species of Mogurnda, most likely the Purple Spotted Gudgeon Mogurnda mogurnda. If you look for photos of this fish on the internet, you will see that it is very variable with some looking quite like yours, others rather different. However, yours shows some typical features, which seem to be characteristic of this species :

there are three dark lines on the gill covers, behind the eye
there is a row of dark spots along the base of the anal fin (underside, in front of the tail)

Yours may still be immature and might develop more colour later.

If it's not this species, it is certainly closely related.