Niue and Tuvalu have voiced their concerns about the release of treated wastewater from the Fukushima nuclear plant.
A "controlled release" started about 1pm on Thursday, Japan time.
Niue Premier Dalton Tagelagi says his country is worried about the release as the impacts to human health and environment are still unclear.
"The majority of Niue are coastal peoples, and the ocean is an integral part of our culture, traditions and livelihoods and we must protect it at all costs," he said.
"This release of treated nuclear wastewater is a transboundary and intergenerational issue, and similar concerns were shared by other member nations at the recent Pacific Leaders Forum."
But Tagelagi said given the release has gone ahead, Niue expects to receive continued monitoring results from Japan to show that the initial discharges are safe, "including continued independent reviews from the International Atomic Energy Agency (IAEA) and others".
In Tuvalu, Minister of Finance and Economic Development Seve Paeniu said he is "concerned, dismayed, disappointed and kind of surprised" that Japan - which has first-hand experience of nuclear material - is now putting nuclear waste into the Pacific Ocean.
"Tuvalu collects roughly about 45 percent of its total income from fisheries and fisheries resource through access licensing fees to foreign fishing fleets coming to fish in our surrounding waters," he said.
"Not only that, the livelihoods of our people are solely dependent on the marine life, on the fisheries resources surrounding our waters."
He said Tuvalu was not able to attend the International Atomic Energy Agency (IAEA) but they will take every opportunity to bring this up with the Government of Japan.
"Also, we would like to hear more clearly from Japan, the basis upon which they made that decision."
Meanwhile, a spokesperson from Pacific Elders Voice said Pacific Islanders and future generations will bear the brunt of Japan's dependence on nuclear energy.
They say the "brazen act" will compound the nuclear legacy in the Pacific and disrespects the region's strong stance against nuclear pollution.
'There continues to be divergent views and responses'
Fiji Prime Minister Sitiveni Rabuka has expressed his support for the release, saying he is satisfied with the IAEA report.
The Pacific Islands Forum has not made a firm stand one way or the other.
But Secretary-General Henry Puna has said he regrets the fact that Japan has gone ahead to start discharging before the Pacific Islands Forum scientific team has verified that it is safe to do so, but stressed that Japan is a soverign nation.
Puna said the Forum Secretariat has worked with all members to pursue different avenues over the last three years to "urge Japan to take all steps necessary to address any potential harm to the Pacific".
He said they have relied on Japan's assurances that the discharge will not take place if it is not verifiably safe to do so, and their commitment to ensuring any release would "not be allowed in a manner that endangers the lives of Japanese citizens or those of the citizens of Pacific Island countries".
He said the leaders have noted the IAEA report and considered the advice of their PIF independent scientific experts over the past 18 months.
"It remains clear however that there continues to be divergent views and responses in the international community and within the Forum membership on this issue," Puna said.
"I recognise the sovereignty and prerogative of Forum members to determine their own national positions."
He said they will continue facilitate communication with the Government of Japan and the IAEA to keep Forum members up to date.
On Friday, protesters in Auckland decried New Zealand's 'convenient' silence on the nuclear waste release at a rally in the city centre, while there were also protests in Suva, South Korea and elsewhere in the Asia-Pacific rim.