The hourly rainfall at the peak of Sunday's dramatic storm was the highest recorded in March since records began in 1884, the Observatory revealed yesterday.
The night saw the first time the black rainstorm signal was used in the month of March since the warning system was introduced in 1992.
The Observatory said 56 millimetres of rain fell between 9pm and 10pm in some areas - a record for a single hour in March. More than 100mm was recorded in three to fours hours in Kowloon and the New Territories, with rainfall reaching 150mm in Yuen Long, Tuen Mun, Tsuen Wan and Sha Tin.
The month of March typically sees just 82.2mm of rainfall, according to Observatory statistics.
Watch:Black rainstorm hits Festival Walk shopping mall in Hong Kong
The Observatory earlier revealed that the 103.1mm of rainfall recorded at its headquarters in Tsim Sha Tsui on Sunday was the fourth highest on a single March day in history.
The storms continued on Wednesday night, when hail again fell in the New Territories and the amber rainstorm warning was issued. The warning was cancelled at 6am yesterday.
Hailstones the size of golf balls fell on Tsuen Wan during Sunday's storm, which disrupted hundreds of flights. There were also long delays on some MTR lines.
The storm washed out celebrations at the end of the rugby Sevens at Hong Kong Stadium, and there was widespread flooding - not least at the Festival Walk shopping mall in Kowloon Tong. In its monthly weather report, the Observatory reported that March had been characterised by "gloomy weather during the first half… and heavy rain episodes towards the end", with only five hours of bright sunshine, all during the first half of the month.
The average temperature last month was 18.7 degrees Celsius, compared to an average of 19.1 degrees for March.
Total rainfall for the month was 207.6mm, compared to the normal figure of 82.2mm - and 99 per cent of that fell in the last three days.
But weather forecasters say brighter weather is expected for today and tomorrow, before thundery showers resume next week.
In June 2008, 145.5mm of rain fell between 8am and 9am in Tsim Sha Tsui, the most in an hour since records began in 1884.