"Robin is an excellent speaker and experienced campaigner who embodies Act's values. He will contest Northland with vigour," Act Party president John Thompson said.
Mr Grieve was third on Act's party list in 2014, and stood in the Whangarei electorate in the past two elections. He owns an avocado orchard and is a consultant and trainer for Farmsafe and has also represented New Zealand internationally three times in Toastmasters competitions.
Labour has confirmed Willow-Jean Prime as its candidate in the byelection. A lawyer and Far North District councillor who had her first child just three weeks ago, she cut Mr Sabin's majority by more than 2000 in the general election in September.
Mana has named Ahipara environmental activist Rueben Taipari Porter as its candidate, while Mel Taylor of Maungaturoto is believed to be running for the Conservative Party.
The Greens will decide whether to contest the byelection early next week. NZ First has yet to decide if it will stand a candidate.
Meanwhile, a series of "grill the candidates" meetings have been organised around the Far North.
Dates and venues are Oruru's Swamp Palace on Sunday, March 8 (2pm), Far North Reap in Kaitaia on Friday, March 13 (1.30-5pm), and Kerikeri's Turner Centre (7-10pm). Entry by koha to cover venue hire.