Conservatives leader Colin Craig says the party decided it would not stand in the Northland byelection because it did not want to split the centre-right vote and help Winston Peters.
Its candidate also had a new baby and was not available, he said.
The party executive met on Saturday and made the decision.
In a surprise that day, National's locals picked Mark Osborne to run for the seat, which the party held at last year's election with a 9300 majority.
Most predictions, including from National pundits, had been tipping party board member and local farmer Grant McCallum.