The Tonga Meteorological Services said a tsunami warning had been put in force for all of Tonga.
A massive underwater volcanic eruption off the island nation of Tonga has been felt across the south-western Pacific, with many tsunami warnings still in place, including for much of Australia's east coast.
The Bureau of Meteorology said the Hunga Tonga-Hunga Ha'apai volcano erupted at 3.10 pm AEDT on Saturday, and tsunami waves have been observed as a result of the eruption.
Tsunami warnings were issued across much of the Pacific, including New Zealand, Canada, the US, and Australia's Lord Howe, Norfolk, and Macquarie Islands.
Witnesses in Tonga said the blast shook the earth for several minutes and sounded like rumbling thunder.
Video filmed by locals shows waves more than a metre high crashing into houses.
In Vanuatu, the authorities have received reports of waves up to two-and-a-half metres high.
Tsunami waves nearing 1 metre have been recorded on the Australian mainland.
"The size of these waves means the threat is for the marine environment for the east coast of Australia, and for land on Lord Howe Island and Norfolk Island; however the situation will be closely monitored and warnings updated as required," the BOM said in a statement.
"People in land-warning zones are strongly advised to move 1 kilometre inland or go to high ground at least 10 metres above sea level.
"While evacuations are not necessary for marine warning zones, people in these areas are advised to leave the water and move away from the immediate water's edge."
The statement said tsunami waves are more powerful than beach waves of the same size.
"There will be many waves and the first wave may not be the largest," it said.
The BOM issued marine warnings for parts of Queensland, New South Wales, Victoria and Tasmania.
Early on Sunday morning, the NSW State Emergency Service (SES) ordered evacuations of low-lying parts of Lord Howe Island.
Big waves on Gold Coast and Twofold Bay
According to the BOM, large waves have been observed on Australia's east coast, with Queensland's Gold Coast seeing waves of up to 0.82 metres and Twofold Bay in southern NSW recording 0.77 metre waves.
Norfolk Island has observed waves of up to 1.27 metres.
Tsunami impacts of unusual currents and waves have also been recorded at Lord Howe Island and Derwent Park in Hobart.
New Zealand warning for north and east coasts
New Zealand's emergency management agency issued an advisory on tsunami activity for its north and east coasts.
In the areas, some 2,300 kilometres from Tonga's volcano, we're expected to experience strong and unusual currents and unpredictable surges at the shore.
The Tonga Meteorological Services said a tsunami warning had been put in force for all of the island nations.
Fiji issued a tsunami warning, urging residents to avoid the shorelines "due to strong currents and dangerous waves".
Tsunami waves measuring 60 centimetres in height were also observed by sea-level gauges at the capital of American Samoa, Pago Pago, following volcanic activity in Tonga, the US-based Pacific tsunami warning centre said.
The tsunami threat continues and sea-level fluctuations and strong ocean currents pose hazards along beaches in harbours, the tsunami monitor said in a statement.
The US National Tsunami Warning Center has issued a tsunami advisory for Hawaii and the northwest US, which includes the states of Alaska and California.
Several hours later, a tsunami advisory was issued for Washington state on the north-western US coast, which includes the major city of Seattle.
'Loud thunder' heard in Fiji
Lasting eight minutes, the eruption could be heard as "loud thunder sounds" in Fiji, more than 800km away, officials in the capital Suva said.
Jese Tuisinu, a television reporter at Fiji One, posted a video on Twitter showing large waves washing ashore with people trying to flee in their cars.
"It is literally dark in parts of Tonga and people are rushing to safety following the eruption," he said.
The eruption, located about 65 kilometres north of Nuku'alofa, caused a tsunami measuring 1.2 metres, the BOM said.
The federal government is "monitoring the situation and "ready to provide support to Tonga if requested".
"Tonga is part of our Pacific family and our thoughts are with the entire community dealing with the impact of the volcanic eruption and tsunami," a Department of Foreign Affairs and Trade spokesperson said.
"Initial assessments are still underway and DFAT is working to ensure Australians in Tonga are safe and accounted for."
The Japan Meteorological Agency has also issued tsunami warnings for the eastern coastal regions, with waves of 1.2 meters reaching Amami Island in the south of the country.