Timothy Raymond Mathieson, 66, appeared remotely in Melbourne Magistrates Court on Thursday by video link from his lawyer's office.
The court was told Mathieson, a former hairdresser, was initially facing three charges.
He sat behind his barrister Brad Penno, who said the matter had resolved and Mathieson will plead guilty to one sexual assault charge.
That charge involves Mathieson sexually touching a woman by "sucking her nipple without her consent" in circumstances where he did not reasonably believe she had consent, court documents reveal.
The incident occurred on March 13 last year in Brunswick East, about two weeks before Ms Gillard told the Adelaide Advertiser the couple had split more than a year earlier.
The couple were together for a decade and lived at the The Lodge and Kirribilli House throughout Ms Gillard's three-year term as prime minister.
Two other charges were withdrawn on Thursday, but further details of those offences were not aired in court.
Mr Penno asked for Mathieson to be handed a diversion, which would mean he escaped a criminal conviction and had the case diverted out of the justice system.
The prosecution asked for two weeks to assess whether a diversion would be appropriate.
Mathieson will return to the court on August 31 for a further mention.
He declined to speak to reporters as he left his lawyer's office on Thursday afternoon and was driven away in a white Nissan Navara ute.
When Ms Gillard was prime minister, Mathieson was colloquially known as the 'first bloke', a reference to the tradition of the wives of US presidents being called first ladies.
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