The Seniors Card does not expire. If the numbers wear off, you can ask for a replacement card.
You can contact the NT Seniors Card office with all queries including;
The NT Seniors Card office is open to the public 9am – 3pm, Mondays to Fridays (except public holidays and during the Christmas close period) or by prior appointment. The NT Seniors Card office occupies space in a convenient CBD location at COTA NT, Spillett House, 65 Smith Street, Darwin