The saying slow as a wet week took on a whole new gravitas last month as the NSW coast experienced its wettest seven-day period since national daily records began in 1900.

Rainfall over the area of NSW which drains into the Tasman Sea averaged a record-breaking 252.9 millimetres in the seven days to March 24. Five consecutive days of between 30 and 50 millimetres was exceptional and resulted in the record weekly total.

“Events in which the full length of the NSW coast experiences significant heavy rain are rare, reflected by the fact that the week ending 24 March 2021 was the wettest on record averaged over coastal NSW,” a Special Climate Statement issued by the Bureau of Meteorology today says.

The weekly average total for the whole of NSW, including coast and inland areas, was 102.2 millimetres, the third highest on record after larger weekly averages in 1974 and 2012.

During the last half of March, heavy rainfall extended from central Australia to northern inland NSW, resulting in significant flooding on some inland rivers and most coastal catchments in NSW and some parts of south-east Queensland and eastern Victoria.

“The stand-out difference in March 2021 is the heavy rainfall in inland NSW, where many areas received three to four times their average monthly total during March 18-24,” the Bureau says.

Flooding reached record heights on the Hastings and Manning Rivers while the Hawkesbury–Nepean catchment in Sydney experienced its most significant flooding for more than 30 years as extreme rainfall struck many parts of eastern and central Australia in the second half of last month.

A high-pressure system in the Tasman Sea and a low-pressure system off north-west Australia fed a large volume of moist tropical air into eastern Australia.

“As the climate warms, Australia's heavy rainfall events are expected to become more intense as moisture in the atmosphere increases by about 7% per degree of warming,” the Bureau says.

“Rainfall changes in specific locations, especially areas with complex topography such as coastal NSW, will have a higher level of uncertainty due to changes in the occurrence, strength and position of synoptic weather systems.

“For the Australian continent, there is evidence that a higher proportion of total annual rainfall has come from heavy rainfall days in recent decades,” it says.

March rainfall totals broke records in many locations, according to the special report which crunches the numbers in full detail on the extraordinary event, which the Insurance Council of Australia (ICA) declared a catastrophe and is expected to result in about $1 billion in insured losses.

It was the wettest March on record for parts of the NSW mid north coast, Sydney and the north west slopes regions. March rainfall overall for the state was 136.3 millimetres – 153% above the 1961−1990 average and marking the second-wettest NSW March on record after 1956, and the wettest month since January 1995.

Daily totals exceeded 150 millimetres somewhere in NSW on each day from 18 to 24 March, for example Moree which received more rain than it had in all of the severe drought years in what was its wettest day since 1888.

A number of NSW coast sites had four-day totals exceeding 600 millimetres.

“One of the most significant aspects of this event in coastal NSW was its persistence, which resulted in many very high multi-day rainfall totals,” the Bureau says.

Almost the entire NSW coast experienced heavy rain, contrasting with typical rainfall patterns associated with either coastal lows or East Coast Lows which normally bring heavy rain in a relatively small area near the southern flank of the low but are relatively dry to the north.

“The most exceptional aspect of the event was the spatial extent of the heavy rainfall, both in coastal and inland areas,” the report notes.

Averaged over all of NSW, March 23 recorded 38 millimetres to be the second-wettest day on record. Two- and three-day totals were also the second highest ever.