The shifting political fortunes of Auckland MPs affected by two proposed electorates are revealed in a Herald data investigation.
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The electorates of Kelston and Upper Harbour are created by a proposed shakeup of electoral boundaries.
The Kelston electorate is expected to be safe for Labour but its creation could pose problems for the party in the neighbouring electorates of New Lynn, Te Atatu and Mt Albert.
Labour MP Phil Goff concedes the proposed boundary changes will mean he will compete for a drastically different Mt Roskill that has gained strong National-leaning polling places from Maungakiekie, in turn making it tougher for National's Sam Lotu-Iiga to retain his seat.
The Herald "mashup" of the proposed electoral boundaries combined with the party votes at 2650 polling places from the 2011 election reveals a reshaped political landscape.
The proposed Upper Harbour electorate is strongly National overall, with a few polling stations on the southern boundaries that could be considered marginal for either of the main parties.
Most of the polling places in Kelston - which would replace Waitakere, currently held by National's Paula Bennett - are strongly Labour-leaning. These polling places were formerly inside the New Lynn, Te Atatu and Mt Albert boundaries. Consequently, the three electorates end up with more evenly split voting patterns, when earlier they were more favourable to Labour.
Te Atatu, currently held by Labour's Phil Twyford, loses a moderately Labour-leaning polling place and adds a strong National-leaning one. Auckland Central loses polling places that are either marginal or Labour-leaning.
Another big change is in the Mt Roskill electorate, which loses a number of Labour-leaning polling places to gain strong National-leaning ones.
A crude calculation based on party votes within the proposed new Mt Roskill electorate shows National with a majority of about 1500 over Labour. However, this only takes into account the party vote and only indicates that Phil Goff - who has held the seat since 1999 - will have a closer fight on hand.
The former Labour leader said the party vote in the proposed Mt Roskill electorate will strongly favour National.
"But I'm confident of winning my electorate even if it is with the reduced majority as compared to the last election."
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He also reckoned that the new proposed boundaries are more likely to cause an upset for a National electorate than a Labour one.
Rob Salmond, a former Labour political director who recently founded data consultancy Polity, said what the data mashup shows is how people would have voted with these new boundaries in 2011.
"We don't know if the voting preferences has changed since then. For example, people in Mt Roskill might vote for National but with Phil Goff as candidate, the same people could vote for Labour now.
He said the change in boundaries affects individual politicians more than the political parties.
"It matters which individuals get to call themselves MPs and attract electorate funding for their parties."
He said the change in electorate boundaries does not actually matter in terms of the party vote apart from whether National does a deal with Colin Craig, the Conservative party leader.
Before the proposed boundaries were released, there was speculation that National would not stand against Mr Craig in the new Upper Harbour seat - much like the deal done with Act leader John Banks in Epsom in 2011 - to gain a coalition partner.
But this was scotched when Paula Bennett, who will lose her Waitakere seat in the shakeup, said she would stand in Upper Harbour for National.
Pollster and centre-right blogger David Farrar said the changes are finally balanced but overall they slightly favour National.
"Waitakere was the marginal seat and now it's gone. Labour gets a safe seat in Kelson.
"Auckland Central goes National and Mount Roskill has definitely changed a lot."
How the data mashup was created
All the party votes for the 2011 election at each polling place were added up. Readers can switch between the proposed and the previous electoral boundaries online and see the difference. The data mash-up is indicative of which party the new electoral boundaries might favour.
Party vote was used as the basis for as it allows for a better comparison between different electorates. Also, this allows for votes from different electorates at a common polling place to be added up to see local trends.