Kiwiblog has an old article with this: (Colmar Brunton)
Kiwiblog: Clark has appalling ratings and was at 5% for most of the next three years. Her rating lifted to 17% just before the 1996 election. She then polled at between 20% and 30% while National and NZ First had their stormy term in Government.
1996 was a loss for Labour. In part because NZ First went into a National Party coalition. Mainly because Labour lost several seats. We also have your graph saying Clark was at 24% at the time of the following 1999 election which she won.
What does your NZE study data give for Clark in 1996?