A link to a post a short while ago, giving most of the big real estate agents. Have a look on their sites for rentals. Some have drop down menus you can select 'Rental' from, others you can select it on the home page.
http://www.movetonz.org/forum/viewto...=rental+agents
The one I found with a blue rental button was Propertystuff.co.nz. Perhaps this is the one you heard about. Apart from that Realenz is the biggest one, I think.