This table from Wikipedia shows all the polling results from non-leaked polls. Since Jacinda became leader of the Labour Party, both NZ First and Greens have had their support effectively halved, to Labour’s benefit.
The Green Party support didn’t drop until Jacinda became leader and Metiria resigned. Metiria’s admission had no effect on the polls.
While the speed of the Greens losing support to Labour was probably greatly influenced by Metiria’s resignation, NZ First has suffered almost just as much at this point. Everyone would expect NZ First to rapidly lose support if their leader resigned.
Metiria should not be blamed for the Greens loss of support. This is a byproduct of a strong Labour Party, and little more.
For the record, I don’t think Metiria herself should be solely blamed for the faster loss of Green support. The mistake was not telling her story (which the Green leadership backed 100% in the first place), but failing to prepare for the media attention the story attracted. As James has said, they only recieved about 1% of the entire media coverage in the 2014 election, and this was a (successful) attempt at increading their share. Unfortunately, whether naive, media inexperienced, optimistic, or all of these, the party (not solely Metiria) failed to preemptively tie up any loose ends and get the media strategy straight. That is the mistake the Greens made, and that is the only mistake.