Defence boss to leave public service

One of the nation's most senior public servants has resigned after nearly half a century of service.

Dennis Richardson will finish up as head of Defence in May, after five years leading the key department.

Over a 48-year career, he also led the Department of Foreign Affairs and Trade and served as Australia's ambassador to the United States and Director-General of Security, working with 12 different prime ministers, including as Bob Hawke's principal adviser.