"This new one-way travel policy will significantly expand the potential workforce available for those experiencing labour shortages," Ms Ardern said.
"Our closed border has been critical to keeping Covid out and keeping our economy running but... we know our agriculture sector is experiencing challenges," she added.
"We've heard the call from primary sectors and others to bring in additional workers in a safe way and we think that is now possible."
Tonga, Samoa and Vanuatu have largely kept COVID-19 at bay.
There have been four cases in Vanuatu, three in Samoa and none in Tonga.
New Zealand, which has recorded just 26 COVID-19 deaths in a population of five million, opened a trans-Tasman travel bubble with Australia in April.
It was hailed as a major milestone in restarting the global travel industry, but the arrangement was suspended last month when a Covid-outbreak spread through Australia.
Opposition leader Judith Collins said the decision to open up to seasonal workers from three relatively risk-free countries was a welcome move but did not go far enough.
"The question now is, if we can bring in (seasonal) workers without them having to undergo quarantine once they enter New Zealand, why can't we prioritise other people from these three countries for quarantine-free travel?" Ms Collins asked.