Taiwanese lawmakers and fishermen headed to an island in the disputed South China Sea Wednesday to protest at an international tribunal ruling which undermined Taipei's claims there.

Eight lawmakers from the ruling Democratic Progressive Party and the opposition Kuomintang (KMT) boarded a military plane to the Taiwan-controlled Taiping island in the Spratlys archipelago.

Five fishing boats decorated with Taiwanese flags and banners reading "Protect fishing rights, safeguard sovereignty" also set sail to Taiping from southern Pingtung county to protest at the perceived threat to fishermen's livelihoods.

The fishermen waved to onlookers, who shouted their support and set off firecrackers as their boats departed. Those boats will arrive in five to six days' time.

The protests come after the Permanent Court of Arbitration in the Hague ruled last week that China has no historic rights to its claimed "nine-dash line" in the South China Sea and had violated the Philippines' sovereign rights in Manila's exclusive economic zone.

Crucially for Taiwan it also ruled that Taiping, the largest island in the Spratlys chain, was legally a "rock" and not entitled to its own exclusive economic zone, undermining Taiwanese claims to waters surrounding the island.

Taiwan last week sent a warship to the South China Sea "to defend maritime territory", with President Tsai Ing-wen rallying troops on the frigate's deck a day after the ruling.

"The ruling is absolutely unacceptable. It is necessary for us to visit Taiping at this time to show the international community that it is an island, not a rock," said KMT lawmaker Johnny Chiang, who was part of the protest visit.

The lawmakers watched a display of combat skills by the coastguard stationed on Taiping as well as visiting facilities that show the island is self-sufficient. They are due to return Wednesday afternoon.

When the fishermen arrive in Taiping they will receive drinking water from the island in a bid to prove it is more than just a rock and is fit for human habitation, a spokesman for the group said.

Taiwan last year inaugurated a solar-powered lighthouse, an expanded airstrip and a pier as part of efforts to strengthen defence capabilities on Taiping.

There is also a farm, water well, hospital and temple on the island.

Taiping island is 0.51 square kilometres (0.19 square miles) and most of its inhabitants work for the coastguard, which has about 160 staff there.

Each year about 200 fishing boats operate in the waters near Taiping island.

The Spratlys are also claimed in part or whole by China, Vietnam, the Philippines, Malaysia and Brunei.

Taiwan's government rejected the arbitration court's ruling, saying it "severely jeopardised" Taiwan's rights.

It said the judgement has no legally binding force since the tribunal did not formally invite Taipei to take part in its proceedings or solicit its views.

Taiwanese authorities have said they will continue to send aircraft and ships for patrol missions to the region and expel any foreign boats that enter its 200-nautical-mile exclusive economic zone despite the ruling.