Prime Minister John Key could face a battle for his seat from NZ First leader Winston Peters.

Back on the campaign trial yesterday, Mr Peters spoke to students at Victoria University and later said that standing in Helensville would be "an interesting proposition".

He would not contest his former seat of Tauranga – which he lost in 2008 – saying he was settled in Auckland. Campaigning in Mr Key's seat would force the prime minister to "turn up and have a debate".

A spokesman for Mr Key said yesterday: "It's a democracy, good luck to him."

Although Mr Key won the seat in 2008 with a majority of more than 20,000 – standing against him could be an astute move for NZ First. TV networks traditionally have had a firm rule of not including leaders from parties outside Parliament in televised debates.

By taking on the prime minister, Mr Peters would ensure coverage of candidates' debates in the electorate.

Mr Key ruled out working with NZ First as a support party before the last election. He said recently that he would consider his stance closer to the election.