Polls opened Saturday in a cliffhanger New Zealand election pitting conservative Prime Minister Bill English against charismatic young rival Jacinda Ardern.

The campaign has been the most volatile in recent memory, with momentum swinging from English to Ardern and then back again.

"This election is going to be really close... it’s a drag race between the two big parties," English said on the hustings Friday, conceding a large undecided vote could prove key.

No party has claimed a majority government in New Zealand’s 120-seat parliament since proportional voting was adopted in 1996 and this election is unlikely to change that.