Meat and poultry have a higher chance to pass on communicable diseases that could devastate the local industry. Fish products don't have that issue, that's why the discrepancy in rules.
Processed packaged foods are mostly OK but try to avoid things with meat, eggs or dairy to be safe. And chocolate does not need to be declared, in fact Aussie customs wants travellers to stop declaring chocolate😆, it is considered the most over declared safe item.