Additional tenants allowed?

Our neighbours have four people in their 2 bedroom unit. They have recently had a fifth member staying with them, for over six months now, so we assume that person is permanent. Is there a maximum number of tenants per unit, and do the tenants need to inform the landlord if they are adding a new member? We also have noted that this one unit now has the same number of people living in it as the other 4 units combined. What can we do about sharing water/utility costs more evenly? This is in South Australia.