Mr. Joyce will also no longer serve as acting prime minister during Mr. Turnbull’s White House visit next week, a decision critics said suggested the prime minister had lost confidence in him. The finance minister, Mathias Cormann, will step in instead.
Mr. Turnbull had appeared to resist the idea of a formal ban when it was suggested last week by Cathy McGowan, an independent politician. But on Thursday he said that the five-year-old code of ministerial conduct did not adequately speak to modern workplace values.
“We must recognize that whatever may have been acceptable or to which a blind eye was turned in the past, today, in 2018, it is not acceptable for a minister to have a sexual relationship with somebody who works for them,” Mr. Turnbull said. “It is a very bad workplace practice.”
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Ms. McGowan said Thursday that although she welcomed the prime minister’s announcement, the government should consider a code of conduct for all those who work in Parliament.
Mr. Joyce, 50, on Tuesday acknowledged his relationship with Vikki Campion, his former media adviser. Though he has spent much of the past week offering remorse to his family, he has denied ministerial misconduct and refused to resign.
Mr. Turnbull said it was up to Mr. Joyce whether to step down. If he does, Mr. Turnbull’s coalition government will lose its one-seat parliamentary majority.
The Australian tabloid newspaper The Daily Telegraph reported the affair last week alongside a photograph of a pregnant Ms. Campion, fueling a fierce debate about privacy, sex and the news media.
Sharri Markson, the national political editor at the newspaper, said she was surprised by the initial pushback from other news outlets on whether the story was in the public interest.
The debate has since grown to include questions about gender and abuse of power.
“The Australian media in particular has protected politicians from this sort of exposure for many years,” Ms. Markson said.