The 29-year-old, who now lives at Beacon Hill on Sydney's northern beaches, initially planned to fight the allegations, with her lawyer telling the court she was struggling with mental health and drug issues at the time.

But she later pleaded guilty to the charges, the more serious of which carries a maximum 10-year jail sentence.

Video played at the hearing from before her arrest showed Goodson speaking with border officials about her plans to study and attempting to explain why she brought a firearm in her bag.

"I'm trying to get into clown school," she told them.

"I've heard other (they) make a pretty decent living."

Goodson said she had done little research on gun laws in Australia because she was "more focused on packing other stuff".

But she added brief online searches led her to believe it would be acceptable as long as the weapon was kept in a case.

"You know when you Google something and it gives you the drop-down menus? It was off those,” she told border force officers.

The California resident said she bought the gun on an auction website and kept it under the passenger seat of her car for protection.

Goodson added she had a permit for concealed carry, which was easy to get in the US.

"I'm going to be honest and say I'm scared of shooting it, but I know in Los Angeles, if you just brandish it people will scatter," she said.

Goodson said she had no intention of using the firearm until she was a resident of Australia with a permit and would "just keep it lying around".

She was previously granted bail on conditions that she handed in her passport, reported daily to Bondi police and did not possess a firearm or ammunition while in the country.

Goodson is due to be sentenced on November 15.