I talked to the guys and this is what they've said: "This application is taking the details you provide for your account and retreiving balance and usage details from Your Telecom. As such if we add new plans or change layout etc this can break the application. We recommend you use Yor Telecom for Mobile to ensure that you receieve all of our usage details. Just got to http://m.telecom.co.nz/yourtelecom from your handset. If you access it via 3G log in is seamless and its free to use."
I know this isn't an app but I have also passed on details to the product team to let them know that you guys would like an application.