Student or Learner
this was the first occasion on which I have seen her
this was the first occasion on which I saw her
First, I would prefer "during which" rather than "on which" but I really can't tell you why I prefer that.
But to your question of "have seen" or "saw" - the "saw" expression suggests a singular moment in time. "I saw you fall down" - not "I have seen you fall down" - unless of course, the person you saw is habitually drunk and falls down a lot.
On that one occasion, that specific moment in time, that is the first time you SAW her. You may HAVE SEEN her several times since then.