Listeners to ABC Radio regularly hear correspondents and experts discuss all sorts of different topics, but in the late 1930s there was one commentator who was a little bit different.

Annette Wagner was a world traveller, a fashion expert, a media identity on the ABC and commercial radio in 1938 and 1939 — and a Nazi spy.

"We don't quite know who she was," Sydney historian Greg Clancy told Linda Mottram on 702 ABC Sydney.

"She was born in Switzerland, she grew up in England, she married a Frenchman, and she came to Australia in 1938, ostensibly to recover from a medical condition."

Though she was in Australia for less than two years, she quickly built a media profile featuring on 2NC (now 1233 ABC Newcastle) offering listeners fashion advice and travel tips.

"She was very well travelled, well educated and intelligent," Mr Clancy said.

Ms Wagner first lived in Cockle Creek, in the western suburbs of Newcastle, before she started staying in Mosman and appearing on Sydney radio 2BL (now 702 ABC Sydney) and 2GB.

Mr Clancy's research shows that, in addition to arranging meetings with Nazi sympathisers through her programs, Ms Wagner may also have embedded encoded messages in the broadcasts.

"When she commenced her program, she would read out phoney cables from Paris, in French," Mr Clancy said.

"This was a strange thing to do considering most people couldn't speak French, but it was suspected that she might have been slipping little messages into the French."

While this practice was certainly unusual, it was not technically illegal as it was before World War II had broken out.

"She did a spy flight over Newcastle Harbour and took accurate aerial photography of the port and the militarily significant BHP steelworks, but it wasn't strictly illegal," Mr Clancy said.

Mr Clancy's uncle was the unwitting pilot of that spy flight.

"I believe the intention for her was to establish herself as an identity, and to continue her work once the war started," he said.

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Ms Wagner had, however, already attracted the attention of Australian military intelligence.

"She was a little too keen on watching military exercises, so they opened a security file on her and kept her under 24-hour surveillance at times," Mr Clancy said.

"When the war started all non-citizens were taken off the radio, for obvious security reasons.

"She was furious ... so she set about trying to get herself back on the radio, arguing that she was born in neutral Switzerland and was married to a Frenchman and therefore was not a threat."

However, security services had more than enough intelligence on her to keep her off-air.

Ms Wagner left Australia in January 1940, eventually returning to France where she worked for Nazi intelligence.

She was arrested by French police in 1946.

During an interrogation at Paris police headquarters, Ms Wagner jumped to her death from a third-storey window.

Mr Clancy has published a book about Ms Wagner, titled Hitler's Lost Spy: The True Story of a Female Spy in Australia.