The federal government has urged people - particularly adults - to get vaccinated against diphtheria, as the outbreak spreads across the NT and WA.

One person is believed to have died from diphtheria in the Northern Territory, in what would be the first fatal case of the infectious disease in almost a decade.

The vaccine that prevents this "ancient" disease is freely available. Experts say its resurgence is alarming.

Health authorities say almost all cases involve Indigenous Australians