The worst rains to hit Baghdad in 30 years left four people dead and many of the Iraqi capital's residents struggling to cope with heavy flooding on Wednesday as the government declared a national holiday.

Patients reported long and difficult journeys to hospitals, shopkeepers complained of a lack of business and several roads were immersed in water, in some cases as much as waist-high.

In the predominantly-Shiite northeastern district of Sadr City, the area surrounding Fatima al-Zahra hospital was completely flooded, and patients said trips that normally took 15 minutes had taken them as long as two hours.

"They told me I need to go and get an ultrasound from outside the hospital, but we do not know where to go, we are afraid the roads will be flooded and we will not be able to come back," said a woman who identified herself only as Umm Laith, or mother of Laith.

The 38-year-old said her sister-in-law, who was nine months pregnant, took two hours to reach the hospital earlier on Wednesday.

A medic who spoke on condition of anonymity said four family members -- two men and two women -- were killed and another woman injured when their house in the northeastern outskirts collapsed Tuesday night due to the heavy rainfall.

The Iraqi Meteorological Organisation reported on Wednesday that 6.75 centimetres (2.56 inches) of rain fell in Baghdad a day earlier, which its chief said was the highest such figure in 30 years.

The heavy rain spurred the government to declare Wednesday a national holiday, the fourth time this year it has been forced to do so because of bad weather. The other three were due to heat during Iraq's boiling summer.

The rain also affected shopkeepers who saw dramatically lower sales as a result of the flooding.

"I did not do any work since the morning because of the flood," said Mohammed, a 35-year-old working in a furniture repair shop in Sadr City.

He said his house in a nearby neighbourhood and the surrounding area were flooded, and complained that he failed to even earn the $8-$12 a day he usually makes.