When shows are made available to view OnDemand and when they expire is all down to the rights we have for the show. This can vary a lot from one show to the next. For example, we can have episodes of Shortland Street up for 6 months, while an Express show playing right after it airs in the U.S. may only be available for 14 days. We sometimes have content with stacking rights too, where we're limited to how many episodes of a show we're allowed up at one time.
We know it can be confusing and frustrating and we're working really hard to extend the length of time we get shows for, and how many episodes of a show we're allowed to have up at the same time. We're also getting more full seasons of shows than ever before!