Torrential rain has lashed swathes of the island since July 28, forcing several thousand people to seek shelter, damaging roads, and shuttering offices.

Maolin, a mountainous district in southern Taiwan, recorded more than 2.8 metres (nine feet) of rain since July 28, the Central Weather Administration (CWA).

That's more than Taiwan's annual rainfall of 2.1 metres last year, according to the agency's data.

It was the first time since 1998 that "Taiwan has seen seven consecutive days each with over 200 millimetres (7.9 inches) of rainfall," said Chen Yi-liang, director of CWA's weather forecasting centre.

The unusually heavy downpours were caused by a low-pressure system and strong southwesterly winds, CWA forecaster Li Ming-siang told AFP.

"The southwesterly winds have brought heavy moisture from the South China Sea to Taiwan," Li said.

Li said southwesterly winds were normally brought by typhoons affecting the island and seasonal rain in May and June.

This time it was caused by Typhoon Co-May pushing southwesterly winds further north as it swept past eastern Taiwan on its way to China, Li said, adding the rain was not linked to climate change.

The average rainfall across the island last month was the highest for the month of July since 1939, the CWA said.

The torrential rain follows Typhoon Danas, which hit Taiwan in early July.

Two people were killed and hundreds injured as the storm dumped more than 500 millimetres of rain across the south over a weekend.

"We rarely encounter a disaster of this scale," Premier Cho Jung-tai said during a visit to a flood-hit area in the southern Tainan City on Monday.

"From Typhoon Danas up to now, we've faced nearly a month of continuous and heavy rainfall."

The week of bad weather left five people dead, three missing, and 78 injured, a disaster official said.

Nearly 6,000 people were forced to leave their homes.

The state weather forecaster expects the rain to ease in the coming days.

Taiwan is accustomed to frequent tropical storms from July to October.

Scientists say human-driven climate change is causing more intense weather patterns that can make destructive floods more likely.