Will ACT return from the grave on 20 September?

Short answer: probably it will.
By announcing his resignation from Parliament (effective appropriately from Friday the 13th), Mr Banks has done all concerned a big favour. The House will have time enough to vote against a by-election in Epsom. (I can't see the Opposition being bloody-minded enough to insist on one.) And John Banks can be scripted out of the political soap opera.
Ever since the brutal take-over staged by Banks and Don Brash, the ACT Party has been headed down-hill. The PR disaster caused by the infamous cuppa tea meeting before the 2011 election cost National a significant percentage of votes and helped to propel Winston Peters back into the House. A one-seat wonder, ACT supported the National-led government, as planned. But it wasn't long before its sole MP was embroiled in controversy over donations to his earlier unsuccessful bid for the Auckland mayoralty. First he had to resign from his ministerial post, and now, found guilty of filing a false return, he has chosen to resign as an MP.
That's all history though. Banks's successors – Jamie Whyte as leader of ACT and David Seymour as candidate for Epsom – are fresh new faces. They are bringing ACT back to its ideological roots: oligarchic rule by private enterprise, fewer public services for the poor and the middle-classes, harsher welfare laws, tougher sentences, unregulated urban development, less government, more market, and a low and flat tax structure. All the stuff that most Epsom voters must love (well, they vote for it), and about enough to earn ACT one per cent of the party vote nation-wide.
Political memories are short. And the election is more than three months away. That's close to eternity in twitter-time. Moreover, National are embarrassed by their lack of likely support parties for post-electoral negotiations. It's highly likely, therefore, that, once again, Epsom voters will get the nudge-and-wink from Mr Key: don't vote for the National candidate, vote for the ACT guy. Doesn't matter who he is. Just vote and forget.
So, as the undead, it's likely that ACT will rise from the grave on 20 September to haunt the House for at least three more gruesome years!