Samoa was being battered by torrential rain on Saturday evening, with authorities receiving reports of flash flooding across the island of Savai'i and homes being evacuated in the capital, Apia.
The country's Meteorological Office said a tropical depression to the west of the country continued to intensifying into a tropical cyclone, and issued several cyclone warnings throughout the day.
In an update on Saturday evening, the office said the system was about 202 kilometres west-southwest of Asau, on the northwestern coast of Savai'i, and about 278 kilometres west-southwest of Apia.
A spokesperson for the office, Luteru Tauvale, said the system was passing to the west of the country's two main islands, but its effects were being severely felt.
He said on Saturday evening that Savai'i in particular was being battered by strong wind and heavy rain, and the office had received reports of flooding across much of the island, including in some villages and towns.
The system was moving southeast towards northern Tonga at a speed of 17 knots and was continuing to intensify, he said, and its slow movement meant Samoa would continue to be experience its effects well into Sunday.
"It might not be a direct hit, but we are being affected by high winds and rain," he said. "What we have seen [is] that if the storm maintains its current travelling speed, it will stay around for 12 to 24 hours."
While there had been no reports of severe infrastructure damage or electricity outages, Mr Tauvale said the wind had blown trees over and caused some minor damage, but the main concern was rain.
RNZ International's correspondent in Apia, on neighbouring Upolo, Autagavaia Tipi Autagavaia, said there was flooding around the Vaisigano River in the town's centre, and shops and a marketplace had flooded.
While the wind had not been "too bad," Autagavaia said persistent rain had fallen in the capital for hours and showed no sign of letting up, which prompted the evacuation of some houses in low-lying areas around the river.
"I can see some houses at the Matautu area have been flooded," said Autagavaia in a phone interview as he stood on the banks of the swollen river, near the popular Aggie Gray's hotel. "Some of the fire and rescue service people are warning people to be prepared and start evacuating themselves to a safe place because we're still under warnings of a cyclone."
People had also been advised to avoid going out to sea with swells forecast to be as high as 13 to 15 feet.
To the east in American Samoa, a tropical storm warning was issued by authorities there for all of the territory's islands.
While the system was about 600 kilometres away from the territory and expected to pass well to the southwest, the US National Weather Service said sustained storm force winds and heavy rain were expected late on Friday night local time (Saturday NZ and Samoa time).
RNZ International's correspondent in Pago Pago, Fili Sagapolutele, said emergency management officials had advised the public to remain alert and to secure loose items, and authorities feared there would be damage.
"One of the concerns with the authorities is the main road. It runs along the mountainside [and] they're concerned that if the ground gets saturated there will be landslides," Ms Sagapolutele said. "There are also several low-lying areas on the main island of Tutuila which are prone to flooding."
The United States Coastguard had ordered all boats and ships to seek sheltered waters, and for commercial and fishing vessels greater than 200 tonnes to remain in port.
A Coast Guard spokesperson said an evacuation of the port at Pago Pago was not anticipated, but the harbour could be closed to inbound and outbound traffic some time on Friday evening.
Tonga and Niue
After intensifying overnight and passing to the southeast of Samoa, the system was expected to move into the northern waters of Tonga.
Authorities there issued a tropical cyclone alert for the Niuas islands on Saturday, but forecasters were warning that the entire country could expect gale-force winds.
The acting director of the Tonga Metservice, Moleni Tu'uoloholo, said it was expected to pass the small, isolated Niuas -- which were warned to prepare for damaging winds, rain and rought seas -- before tracking down towards Niue.
Mr Tu'oloholo says a high pressure system to the southeast of Tongatapu would also cause severe gale force winds for the rest of the country.
"When those two systems come closer together, that's why we have these very strong gale winds throughout the whole Tonga area. We have gale warnings for the whole of Tongan waters," he said.