Darwin just had its wettest September in 39 years, bringing a soggy end to the 'so-called' dry season.

Northern Australia's dry season runs from May until September. During this nearly rainless five-month period, Darwin only averages around 44mm of rain.

By the end of August this year, Darwin had only picked up 2.4mm since the beginning of the season. However, the city then copped a soaking in September and collected more than five times its long-term average for the month.

As of 9am on Monday, Darwin's running monthly total was 86.8mm. This is the city's highest September total since 1981 and its third wettest September on record, with data going back to 1941.

This month's rain brings Darwin's dry season total up to 89.2mm, which is more than double the long-term average and the second wettest dry season of the last decade.

Long-range forecast models have been anticipating a wet end to this year's dry season since June. This outlook was underpinned by warmer-than-average sea surface temperatures near northern Australia and a developing La Nina pattern in the Pacific Ocean.

So, does this mean that Darwin and other areas of northern Australia are about to embark on a wetter-than-average wet season?

The broad-scale climate drivers that lead to an early rainfall onset in Darwin this month will still be at play as we head into the wet season proper. In addition to the developing La Nina in the Pacific Ocean, some computer models also suggest that a negative Indian Ocean Dipole (IOD) could develop later this spring. Both La Nina and the negative IOD typically cause above-average rainfall in northern Australia during the wet season.