John Banks' biographer and former Beehive aide Paul Goldsmith will fight him for the Epsom seat in November's election.
Mr Goldsmith was selected last night as National's candidate from a strong field of five nominees.
Mr Banks, former Cabinet minister and Auckland City mayor, is Act's candidate. Party leader Don Brash handpicked him after rolling the sitting Act MP Rodney Hide from the candidacy.
Mr Goldsmith - a father-of-four, business historian and former Auckland City councillor - wrote Mr Banks' biography in 1997 and Dr Brash's in 2005.
Mr Goldsmith is generally well regarded and eyes will now turn to National Party leader John Key to see what signals he sends Epsom constituents about how to vote.
National is considered unlikely to change its strategy of giving a tacit nod to its supporters to back the Act candidate to increase the chances of its coalition partner making it back into Parliament.
Mr Goldsmith, 40, said he would fight to secure strong support for National in the seat, saying Epsom voters backed the values the party brought to government, including "sensible economic reform" and support for the vulnerable.
Mr Banks said he had known Mr Goldsmith for several years and it was "surreal" to be standing against someone he knew well.
"I moved him from a back room at the Waitangi Tribunal to Wellington, and he worked with me in my mayoral office. I'm close to him and his family."
However, he indicated he expected the accommodation to continue.
Mr Banks said Epsom voters clearly understood "the way the game is played".
"A vote for me is for five or six [Act] MPs or hopefully more, and the National Party candidate will be a member of Parliament anyway.
"I'm looking forward to making sure that John Key and a National Government is elected on the 26th of November."
Early this year, the Prime Minister said National was again likely to focus on the party vote in Epsom - a sign it was again leaving the seat to Act. That was before Dr Brash took over, although the change is unlikely to alter National's stance.
Last night, the field narrowed after three ballots to just Mr Goldsmith and fellow former city councillor Aaron Bhatnagar. The others seeking the candidacy were former city councillor Richard Simpson, former United Future president Denise Krum and former Christian Heritage leader Ewen McQueen.