Taiwanese were Wednesday told to brace for extreme winds and lashing rains as the first typhoon of the season approached, prompting the evacuation of thousands of tourists from offshore islands.

Typhoon Nepartak was blowing gusts of up to 263 kilometres (163 miles) an hour, some 780 kilometres southeast of Taiwan's Hualien city at 0900 GMT, the island's Central Weather Bureau said.

The "super-typhoon" would likely make landfall early Friday morning, the bureau added.

"As the typhoon kept gaining strength and approaching Taiwan over the past three hours, the Central Weather Bureau decided to issue a sea warning at 2:30 pm (0630 GMT)," senior forecaster Chen Yi-liang told reporters.

"The typhoon could bring in strong winds and torrential rains...Residents must heighten their vigilance."

The tropical storm had a radius of 200 kilometres and was moving west-northwest at a speed of 20 kilometres an hour, the bureau added.

Since Tuesday, more than 3,000 visitors have been evacuated from Green Island and Orchid Island, two popular tourist spots off southeastern Taitung county, local government officials said.

The defence ministry said it had ordered around 35,000 soldiers to be on standby.

Super typhoon Dujuan killed three people and left more than 300 injured in Taiwan last year after it pounded the island with fierce winds and torrential rains that uprooted trees and triggered multiple landslides.