Portia Woodman's four tries against the United States have thrust the rampant Black Ferns into the final of the women's Rugby World Cup in Dublin.

Woodman continued a blistering tournament as the unbeaten Black Ferns powered away 45-12 in the semi-finals, setting up a decider against either England or France.

As has been the case all tournament, New Zealand had too much speed and precision for their opponents, scoring seven tries to two.

Five of them came after halftime, having ground out a 15-7 lead at the break.

And four of them went to irresistible winger Woodman, the scorer of eight tries in their pool thumping of Hong Kong last week.

With her team leading 8-7 midway through the first half, the 26-year-old produced a fine solo try to send momentum her team's way.

She was the beneficiary of some fine team attacks in the second spell, joined on the try-scorers' list by first five-eighth Victoria Subritzky-Nafatali, reserve hooker Te Kura Ngata-Aerengamate and second five-eighth Kelly Brazier. Halfback Kendra Cocksedge kicked 10 points.

Four tries came in a dominant final quarter, with the Black Ferns seemingly unaffected by the absence of veteran loose forward Linda Itunu, who was shown a yellow card soon after her introduction from the bench.

"It's taken a lot of hard work from the girls. We're so excited we're in the final," skipper Fiao'o Faamausili said after the win.

"Full credit to the USA team who battled all the way through to the 80th minute. A lot of respect for that team, they really gave it to us."

Faamausili said she was proud of the way the side responded when they were down to 14 players.

"The girls really had to dig deep and we've been training hard throughout the week, our preparation as been on point and I couldn't be prouder of how the girls dug today. We were down to 14 players and had to step in up another notch, so very very happy".

New Zealand have scored 42 tries to five in four games at the tournament, including thumping wins over Wales, Hong Kong and Canada. The average scoreline is 65-7.

They will play for a fifth title in a final in Belfast on Sunday morning (NZT).

Defending champions England, who beat the USA 47-26 in pool play, started as favourites against the French.