Leaders of eight South Pacific countries, including New Zealand and Australia, have signed a treaty to make the region a nuclear-free zone and have asked the five nuclear powers to sign protocols agreeing to ban the use or threat of nuclear weapons and the testing of nuclear explosive devices there.

The treaty, signed Tuesday in Rarotonga in the Cook Islands, would ban the manufacture, acquisition or receipt of nuclear explosives. It also would prohibit testing and stationing of nuclear weapons and the export of nuclear material without strict safeguards.

But a key provision of the treaty allows the transit of nuclear-powered and nuclear-armed warships through the region, and would leave such ships' port visits to the decision of individual countries. This provision would protect, and possibly even enhance, U.S. security interests because it guarantees the maintenance of important international legal safeguards on transit, according to diplomats from the region.

These diplomats emphasized that the pact would not interfere with security requirements of the ANZUS alliance linking Australia, New Zealand and the United States.

But one U.S. analyst said the treaty could be used by the Soviet Union to complicate U.S. access to the Pacific. "It would be logical to assume that it would be used in a way to make it look like the United States is pushing its nuclear policy in parts of the world where they don't want it," the analyst said. "It would not help the U.S. image."

The idea of a nuclear-free zone in the South Pacific, similar to the Latin American nuclear-free zone established in 1967 and to the nonnuclear provisions of the 1959 Antarctic Treaty, has been endorsed by the South Pacific countries for several years. It was originally aimed at France, which still conducts underground nuclear tests on Mururoa in French Polynesia. Many of the Pacific islands fear that the French tests will eventually lead to radioactive pollution of the Pacific and its fish stocks, the main resource for many of the islands. They are also concerned about Japan's long-delayed plans to dump nuclear wastes from its reactors.

It was not until the recent return to political power of Labor Party governments in New Zealand and Australia, however, that the concept gained momentum, according to diplomats from the region. The treaty was drafted under Australia's chairmanship.

During the past six months, the question has become more sensitive because of New Zealand's ban on port calls by U.S. nuclear-armed or nuclear-powered vessels and the resulting U.S. measures reducing defense cooperation with New Zealand.

The five countries that will be asked to sign protocols are the United States, France, Britain, China and the Soviet Union.

The State Department said yesterday it would make no official comment on the treaty or protocol until U.S. officials had a text. But a spokesman said, "We have been given to understand that the treaty does not constitute an endorsement of the New Zealand port ban."

During his trip to the Pacific last month, Secretary of State George P. Shultz expressed appreciation that the treaty was drawn carefully so that the transit of U.S. ships would not be affected. Shultz left open the possibility that Washington may sign the protocol.

The treaty, on its face, does not interfere with any operational requirements of the ANZUS alliance, said one diplomat from the region: "One could also reasonably infer that Australia, which unlike New Zealand, has a fully operational security relationship with the United States, would not have signed it if [Australia] thought it would not be able to fulfill its ANZUS obligation."

In addition to Australia and New Zealand, the countries that signed the treaty are Western Samoa, Tuvalu, Niue, Fiji, the Cook Islands and Kiribati (formerly known as the Gilbert Islands). New Zealand Prime Minister David Lange, spokesman for the forum, said the five other members -- Tonga, Papua New Guinea, Vanuatu, Nauru and the Solomon Islands -- had endorsed the treaty and were expected to sign it within a few months following constitutionally mandated procedures at home. The treaty would go into effect after ratification by eight countries, diplomats said.

However, Vanuatu Prime Minister Walter Lini said later that he would not sign the treaty because it is too weak and "as far as we are concerned is not practical."