John Key says Winston Peters simply wants to "give the Government a black eye" and is not interested in the people of Northland.
The veteran New Zealand First leader is standing in the Northland by-election, with a Colmar Brunton poll for TVNZ's Q&A showing he has taken an early lead in the race.
Speaking on TVNZ's Breakfast show today, the Prime Minister said Northland-born Mr Peters was only turning his sights on the region to be disruptive.
"Today he cares so much after 40 years in politics about Northland," Mr Key said.
"Why is he doing that? It's pretty obvious to see, he wants to give the Government a black eye, be disruptive, that's what he is interested in.
Labour leader Andrew Little gave a strong hint to left-leaning voters yesterday that they can be "realistic" and back Mr Peters in the race, with the poll showing the New Zealand First leader would win easily if Labour's Willow-Jean Prime was not in the running.
The seat was left vacant by the resignation of National's Mike Sabin.
The Government is expected to make an announcement around road funding in Northland today, with its candidate Mark Osborne saying transport was one of the major challenges in the region.
"With my current role I'm travelling in order of 1000km around the district per week," he told Breakfast.
"I've got a really good understanding of the challenges and issues we face, particularly around the roading infrastructure and roading network that will make a real difference to Northland's economy and safety."
At his campaign launch Mr Peters said the people of Northland needed to "send a message" to the Government