Australia has recorded its first diphtheria death in almost a decade as the country grapples with the worst outbreak of the vaccine-preventable disease in decades.

In March, the Northern Territory (NT) declared an outbreak of diphtheria with cases also in Western Australia, South Australia and Queensland. Cases started rising in late-2025 with a sharp increase in February.

This year, there have been 245 cases, marking the largest outbreak in Australia since 1991, mainly in remote Indigenous communities.

On Tuesday, NT's health minister said autopsy results from an overseas lab found diphtheria was the cause of a man's death in April at Royal Darwin Hospital, the first such case since 2018.

In recent weeks, the government has ramped up vaccination efforts in areas most at risk and the number of new cases was now falling, health officials said on Tuesday.

"Our government has taken this situation very seriously, and we are working hard to understand the causes and working to contain the situation," NT Health Minister Steve Edgington said.

Since 30 March, there have been 10,407 vaccinations, he said.

Between January last year and May this year, the NT reported 163 diphtheria cases with 48 respiratory cases and 115 cutaneous cases, which is spread via skin contact.

In March, health officials in Western Australia (WA) confirmed two cases of respiratory diphtheria, the first time in more than 50 years that WA had recorded such cases.

Sixty per cent of the cases this year are in the Northern Territory, followed by Western Australia with about 36%, with a few cases in South Australia and even fewer in Queensland.

Authorities are urging affected communities to update their vaccinations, especially teenagers and adults who need to get booster shots.

Health officials in the NT have set up pop-up clinics in Darwin, Katherine and Alice Springs to raise awareness of the vaccination campaign, as vaccines are the "most important measure for preventing, protecting and reducing transmission," NT Health said.

Both strains of diphtheria - respiratory and cutaneous - are preventable via a vaccine, which is usually given to children - five doses between two months and four years old - with a booster between 12 and 13 years.

Respiratory diphtheria often starts with fever or chills, a sore throat that can lead to breathing and swallowing difficulties, and be life-threatening.

Cutaneous diphtheria usually causes infected sores or ulcers on exposed parts of the body which are slow to heal but rarely lead to severe illness.

It is understood that the last reported diphtheria death was in 2018, according to the national broadcaster ABC.

Last week, Australia's Chief Medical Officer Prof Michael Kidd declared diphtheria a communicable disease incident of national significance.

The government also announced a AU$7.2m package to boost vaccinations and resources in affected areas.