The Department of Home Affairs said while it does not comment on individual cases, all non-citizens entering Australia must meet character requirements set out in the Migration Act. The reasons a person might fail the character test include a criminal record or a determination they might a risk to the community, according to the department.
Prime Minister Scott Morrison said the decision was for Coleman, who was sworn in as immigration minister on Tuesday.
Labor foreign affairs spokeswoman Penny Wong said the government should be transparent about the reason, if Manning is denied entry.
Amnesty International accused the government of trying to silence Manning. "By refusing her entry, the Australian government would send a chilling message that freedom of speech is not valued by our government," Amnesty International national director Claire Mallinson said in a statement.
Lawyer Greg Barns, who has represented Wikileaks founder Julian Assange, said people with criminal records have been allowed into Australia in the past.
He said no one would seriously suggest Manning was a risk to the Australian community.
Immigration New Zealand expects to make a decision by Friday on whether to grant her a "special direction" visa. She doesn't qualify for entry otherwise because she has a criminal conviction within the last 10 years for which she had a sentence exceeding 1 year, according to the country's visa rules.
New Zealand's center-right National Party opposition is urging the government to decline her visa request. If the decision goes against Manning, she can have it reviewed by the immigration minister.
She is due to speak in the Australian city of Melbourne on September 7, the New Zealand city of Auckland on September 8, the New Zealand capital Wellington on September 9 and the Australian city of Brisbane on September 11.
- AP