Campaigning has begun in earnest for the Mana by-election even though a date has not yet been set.
Prime Minister John Key and Labour Party leader Phil Goff were in the electorate near Wellington on Thursday supporting their candidates to succeed Labour's retiring MP Luamanuvao Winnie Laban.
Labour candidate Kris Faafoi believes the rise in the goods and services tax to 15% on 1 October will be a factor in the by-election.
Mr Faafoi says many people in the electorate are struggling and the GST rise will hurt them, even as income tax rates are cut.
Mr Key acknowledges the tax package will be a backdrop to the by-election but is confident people in the electorate will benefit as much as taxpayers elsewhere in the country.
National's candidate, list MP Hekia Parata, says her focus during the campaign will be on education and the economy.