Making sense of apartment rent

Sean asks:
(updated on Friday, February 15th 2013)

I am currently renting a one-bedroom apartment in the Auckland CBD. It is fully furnished and being managed through a property management company. I moved in to the apartment at $550 per week rent. In my welcome pack, I found a letter to the owner from the property management company (I think it was put in here by mistake) dated a few days before I moved in, stating that the estimated property rent is $400 to $430 per week. So I am being charged $120 a week more than estimated. I have now received a letter stating that they are putting the rent up by another $20 a week saying they have completed a study of rents in the Auckland CBD and the apartment is currently being rented out below its market value. I am trying to make sense of this?