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Chris Penk was last night announced as National's nomination for the safe seat, which has held by the party since it was established in 1978.
Mr Key won with a 18,000-vote majority in 2014. Mr Penk is so confident of victory he is already packing up his property law business.
The 37-year-old, who ran unsuccessfully for Parliament in Kelston in 2014, said he was ``keeping his feet on the ground'' after replacing the popular National leader.
``Obviously John Key is a pretty significant local figure, but I'll be focused very much on the local level.''
The son of a law lecturer and a primary school teacher, he attended Kelston Boys High and earned a Bachelor of Arts degree at the University of Auckland, where he later returned to get his law degree.
After his studies, he joined the Royal New Zealand Navy and later the Australian Navy.
His military career took him to the Pacific Islands, South East Asia and the Middle East, where he helped protect an Iraqi Government-owned oil rig in the Arabian Gulf as part of Operation Iraqi Freedom in 2007-08. He also worked as aide-de-camp to Governor General Dame Silvia Cartwright.
Mr Penk joined the National Party in 2008 and worked as Deputy Prime Minister Paula Bennett's electorate chair in Waitakere, saying he shares her ``Westie'' sensibilities.
In Helensville, he will be up against a high-profile Green Party candidate, sportswoman and broadcaster Hayley Holt.
Labour has selected Kurt Taogaga, an ESOL teacher, as its candidate.
Mr Key resigned as Prime Minister in December and will leave Parliament at the election.