Ms Williams, who moved to Christchurch in February, took 61% of the votes cast in the by-election, called following the retirement of long-serving MP Lianne Dalziel (who is now the Mayor of Christchurch).
Mr Cunliffe laid down a warning to the Government after the win by Labour, putting Prime Minister John Key on notice that voters had demonstrated how unhappy they were with the handling of the Christchurch rebuild.
Mr Cunliffe extrapolated out the win to predict a turning of the tide for the ever-popular Mr Key, even going as far to say Labour was going to win at the election next year.
The win for Labour was undoubtedly a strong endorsement of Ms Williams, but how much should be read into the win?
After all, the Labour leader earlier played down the prospect of the by-election result being a mandate on his own leadership.
But it was: every single vote for and against Labour was a reflection of how the party, and community at large, viewed Mr Cunliffe.
Bringing in former Progressive leader Jim Anderton, himself a superb campaigner when he was Sydenham MP after he held the seat in a by-election he caused by resigning from Labour, was a master stroke.
Mr Anderton worked Ms Williams until she was ''exhausted''.
If she was not exhausted, Mr Anderton said, he would find something else for her to do.
This was a make or break campaign for Labour.
There was much at stake.
First, Labour has had a century-strong grip on the seat and losing in a by-election, when no government has ever taken a seat it has not held in such circumstances, was unlikely if impossible.
Secondly, much had changed in the electorate since the earthquakes.
The vote in the by-election was predictably small and there were several issues around the low turnout.
People who have the ability to move out of the damaged eastern suburbs have done exactly that.
Significant boundary changes will be established before the next election, with Waimakariri and Selwyn growing significantly in population.
Christchurch residents have moved west and north.
Those left behind in the worst parts of Christchurch East will be understandably frustrated at the lack of action on their behalf.
But the feeling is not universal.
Some people have rebuilt and moved on.
And while there is a general frustration with the rebuild, the Government has poured billions of dollars into the city and is likely to significantly increase spending next year in the lead-up to the general election.
It was not an insignificant move by Mr Key on Thursday last to announce nine new emergency service headquarters will be built in the city at a cost of more than $50 million.
When the Christchurch City Council played politics over in-fill housing proposals a week or so ago, Earthquake Recovery Minister Gerry Brownlee warned he would sign the legislation himself if the council did not see sense.
While Ms Dalziel said the delay was deliberate, no-one should underestimate the will of the Government to again win the party vote in Christchurch.
Christchurch Central is likely to revert to Labour, but the party vote is what counts, and Mr Key is the first National Party leader to truly understand its importance.
Mr Cunliffe will go into the Christmas break buoyed by the success in Christchurch.
However, a closer look at what else is happening around the regions should give him food for thought.
South Island regions are starting to feel frustrated at the importance the Government is putting on Christchurch when they believe they are being neglected.
The Government argues funding for tertiary education, research and development and roading is part of its investment into the regions.
True.
The obsession with Christchurch may hurt the Government, but only if Mr Cunliffe can harness the resentment and turn it into support for his party.
So far, there is no indication of that happening.
