How come you don't know how old he is? Whether 8 weeks or 9 weeks? I'm going to go ahead and assume you did not get this puppy from a reputable breeder, otherwise you'd know how old he is. How long have you had him?
This is important and might be a clue to your problem. Some states have laws governing the age at which it is permissible to sell (or import) a puppy and it's mostly not before 8 weeks old. It's generally agreed that puppies should stay with Mom and siblings till 8 weeks old because in those last two weeks is when they learn some bite inhibition. If your puppy was removed before 8 weeks it might make training not to bite more difficult. Add into that a breed meant to nip when it's herding (not sure if Blue Heelers are, but if) and that adds more to your problem.
Your trainer is right, it is good puppy is biting now when he is small and you can teach him proper behaviour with humans. The dog who never learns this young and then bites at an older age is harder and there is more potential for injury. This is right out of an esteemed and acclaimed dog trainer's writings, Dr. Ian Dunbar, who says exactly that. He wrote
The Bite Stops Here
You can find this article on many, many doggy sites on the internet, this link is just one. Dr. Dunbar offers more than one remedy to try.
Good luck. As the owner of a Lab, another strongly mouth oriented breed, as most retrievers are, I feel for you. Well, heck, no I don't. I did and I don't want to again, it hurts.
Oh. The growling. Puppies growl when they play, some of them. Puppy classes, bite inhibition training, NILIF, proper exercise will all help you address that.