Tropical storm Freddy is due to hit the coast of southern Africa again after killing at least 27 people in Mozambique and Madagascar since it first made landfall last month.

One of the strongest storms ever recorded in the southern hemisphere, Freddy may also have broken the record for the longest-lasting tropical cyclone, according to the World Meteorological Organisation, which said the current record is held by a 31-day hurricane in 1994.

Freddy was first named on February 6, 33 days ago.

More than 171,000 people were affected after the cyclone swept through southern Mozambique two weeks ago, bringing heavy rains and floods that damaged crops and destroyed houses, according to United Nations humanitarian agency OCHA.

OCHA on Friday put Freddy's latest death toll at 27, 10 in Mozambique and 17 in Madagascar.

As many as 565,000 people could be at risk in Mozambique this time around in Zambezia, Tete, Sofala and Nampula provinces, with Zambezia expected to be the hardest hit, according to the country's disaster management agency.

Its central region director Nelson Ludovico said the agency was preparing for the storm's landfall in the early hours of Saturday and had moved people to makeshift shelters.

"It's a slow-moving cyclone. This is bad news in terms of rainfall because it means it's hovering quite close to the coast and it's picking up more moisture, so the rainfall will be heavier," Clare Nullis, spokeswoman for the World Meteorological Organisation (WMO), told reporters in Geneva.

The storm is likely to cause extreme rainfall over large parts of Mozambique as well as northeastern Zimbabwe, southeast Zambia and Malawi, she said.

Freddy has set a record for the highest accumulated cyclone energy, a measure of the storm's strength over time, of any southern hemisphere storm in history, according to the US National Oceanic & Atmospheric Administration.

The storm has generated about as much accumulated cyclone energy on its own as an average North Atlantic hurricane season, Nullis said.