Apart from a three year break between 1990 and 1993, Goff has been an MP since 1981, and had an 8000 majority in 2014.

His dominance means Roskill has long been ticked-off as a safe Labour seat.

But - as demonstrated in a Herald interactive published today - the "Goff factor" has obscured a shift within Roskill that makes it more marginal than its history indicates.

Goff's popularity has coped with boundary changes that have added neighbourhoods with more National-leaning voters, and jettisoned those with Labour households.

In 2014, National got 14,275 party votes - 2000 more than Labour - but one quarter of National voters opted for Goff.

Indian businessman Roshan Nauhria, who is running as a candidate for the immigrant-focussed People's Party, told the Herald that Roskill was not a Labour stronghold but a "Goff stronghold".

The Roskill MP's departure to become Auckland Mayor meant the seat was open to capture, Nauhria said.

"People still think Phil Goff votes are going to Labour. That is a misconception. It's not a Labour stronghold, it's a Phil Goff stronghold.

"He built his relation with the people, he built a Goff stronghold here. I tell you today, if Phil Goff is here he will 100 per cent win again."

Parmar is also reaching out to right-leaning Goff supporters, praising the former Roskill MP in an interview with the Herald this week and saying, "I have to work hard to get that Phil Goff vote".

Labour candidate and local board member Michael Wood, who has worked as Goff's campaign manager and is considered the front runner, has spoken of the need to attract National voters who supported Goff.

But his campaign's focus is also making sure Labour voters turned out - byelections generally have lower turnout than general elections, and that tends to favour National.