National will be out to increase its majority in the Botany by-election after slashing Labour's majority in Mana last month.
The back-to-back by-elections are the reverse of each other -- in Mana Labour was defending a safe seat, and in Botany National is defending a safe seat.
Labour retained Mana but there was a 6.6 percent swing to National and Labour's 6155 majority was cut to 1406.
National's Pansy Wong, who resigned from Parliament yesterday over misuse of her international travel allowance, won Botany in the 2008 general election with a majority of 10,872 over Labour's candidate.
Botany is an Auckland electorate and the by-election will be held on March 5.
Neither of the main parties wanted another costly by-election before next year's general election but the only way to avoid it would have been for the Government to call a general election within the next six months.
Prime Minister John Key said he didn't see any reason to go to the polls early, strengthening the expectation that he will wait until November.
Mrs Wong said she was resigning because she was concerned the ongoing allegations over her travel claims were hurting the Government.
She admitted last month that her husband Sammy was involved in a business deal on one taxpayer-subsidised trip they made to China in 2008, and she resigned from the Cabinet.
It is against the rules for MPs' international travel allowance -- a 90 percent subsidy in Mrs Wong's case -- to be used for business purposes.
An independent inquiry investigated her travel claims for the past 10 years and found no other instances of misuse but Labour doesn't accept that and continued to probe into her trips and raise allegations in Parliament.
"The Government has important issues to focus on and they should not have any distraction because of me," Mrs Wong said.
"My family has paid a huge price because of my all-consuming political pursuit ... it is time to turn a page in my life, it is time for me to exit politics."
She said her husband's business interests had been severely curtailed because of her political career, which he had unreservedly supported.
"I'm coming home, Sammy," Mrs Wong said at the end of her valedictory speech in Parliament yesterday, made within hours of her resignation announcement.