Seven people have died and thousands evacuated as Penang was inundated by up to 4m of water after an 18-hour storm that ended yesterday morning.
The state's worst flooding caused its Chief Minister to enlist Malaysia's police and military to help with relief efforts.
At least seven people have died due to the storm, said police. Five of them have been identified.
The remaining two victims are still unknown, though one of them is believed to be a Bangladeshi man who died when his home was crushed by a falling tree. One person, a resident of Bukit Tambun, is reported to be missing.
More than 3,500 people were evacuated from Penang, mostly on the mainland, with another 2,000 in neighbouring Kedah forced to leave their homes.
Winds from the storm - which the authorities said were due to Typhoon Damrey which killed at least 27 people in Vietnam - were so strong that a ferry was washed ashore in Butterworth, the main town of mainland Penang, while a landslide-cum-sinkhole tore up a road in front of a row of newly built luxury houses on the island.
The capital George Town was awash with muddy brown water, as frantic residents rushed to salvage their belongings in the wee hours of the morning.
"The situation has become more critical. Water is still rising, the wind is still strong, and waters are up to 10 to 12 feet (3m to 4m), above roofs in Penang island," Chief Minister Lim Guan Eng said in a Facebook post yesterday, shortly after calling Deputy Prime Minister Ahmad Zahid Hamidi for help at 3.30am. "(Datuk Seri Zahid) has promised to send the armed forces to save Penang island."
In Kuala Lumpur, Prime Minister Najib Razak said the federal government will render aid to any state facing natural disasters, even if it was governed by opposition parties as is the case in Penang, which is run by the Democratic Action Party.
"All this will be coordinated by the National Security Council. We will help the flood victims and also provide aid by determining their needs," he told reporters.
George Town resident Sherilyn Kang told The Straits Times that her family was up all night working to rescue their belongings from 30cm-high floodwaters.
"The water has still not subsided so we can't clean up yet," she said at 10am, about five hours after the rain stopped. "This is the worst in Penang's history," she said.
At a press conference yesterday afternoon, Mr Lim said there was no need to declare an emergency based on discussions with the federal government.
Penang State Minister for Local Government Chow Kon Yeow said there was up to 372mm of rainfall overnight in some areas, equivalent to 11/2 months of rain. Although RM300 million (S$97 million) worth of flood mitigation projects have been approved to improve Penang's drainage system, many have not been completed, he added.
By noon, the flood situation had improved slightly, but bad weather is expected to continue, said the Meteorological Department.
The British High Commission in Kuala Lumpur said yesterday that the visit by Prince Charles and his wife Camilla to Penang this evening was still expected to proceed, but it was monitoring the weather closely.
Meanwhile, the Sijil Pelajaran Malaysia and Sijil Tinggi Persekolahan Malaysia examinations - equivalent to the O and A levels respectively - will begin as planned today.
The flooding in Penang is seen as Malaysia's worst since prolonged storms hit northern and eastern states from December 2014 to January 2015, claiming 21 lives and displacing up to 200,000 people, most of whom were from Kelantan and Terengganu.