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Problem is Labour doesn’t have anyone nearly as charismatic as Adern, who has sucked up so much oxygen as an outgoing and personable leader. It was the same with Helen Clark, perhaps moreso. Hard to see Labour winning from here. They’re out for two cycles in my estimate. Feel free to set a RemindMe!
From an election timing POV; if you're going to resign, now is a reasonable choice. 
\~1 month to shuffle the chairs, \~6 months to re-establish the new PM and cabinet in the media and public mind, then into the election campaign.