Well, even National is left wing by international standards. So the swing hasn't been to the left per se but against the incumbent coalition that has been around for the last couple of terms.
The vote was clearly for change even if that change is a National and NZ First coalition rather than a National and Act and United Future and Māori party coalition.
A 5 seat shift is not 'clearly a vote for change' (2 from National, 1 from United and 2 from Maori and the last parliment had an overhang) especially when Labour has traditionally held the Maori seats and Dunne decided to get out! Its more of the status quo than a clear vote for change.
Labour pinched seats off the Greens and Winston 1st primarily because the "bludger scandals" that followed Winston and Turia and scared some people back to the mothership.
Go look at the mix at the last election vs this one. There is no vote for change here. All that is is reshuffling the chairs on the deck but its clearly not a vote for change