Australian woman caught sneaking Mexican fiance into US

US border officers have foiled an Australian woman's plan to sneak her Mexican fiance illegally across the US-Canadian border.

Sarah Louise Branch, 37, from Sydney, entered a guilty plea in the US District Court in the northeast US state of Maine on Tuesday to a charge of encouraging and inducing the entry of an illegal alien into the US.

Branch is being held behind bars by the US Marshals ahead of her sentencing

She faces a maximum five years in prison and a $US250,000 ($A346,400) fine.

US federal prosecutors told the court Branch and Benigno Godinez-Cortez, 46, hatched a plan involving the couple separately entering the US from Canada on July 10 and meeting up later.

Branch drove to the US border entry point at Calais, Maine, in her Honda Odyssey van and Godinez-Cortez, just 800 metres away, illegally crossed on foot by walking along a train trestle.

Eagle-eyed US Custom and Border officers quickly thwarted the plan.

After Branch showed officers her Australian passport, they inspected her vehicle and found male clothing, Godinez-Cortez's expired Mexican passport, photocopies of his valid Mexican passport and a one-year apartment rental lease agreement signed by the couple.

Branch initially told border officers Godinez-Cortez had flown from Canada back to Mexico but as they questioned her, an alert was issued for a possible illegal immigrant crossing along the nearby train trestle.

They found Godinez-Cortez waiting in a Tim Horton's restaurant in Calais.

Gordinez-Cortez had been ordered to leave the US in 1994 and did not have permission to return.

When officers challenged Branch about not telling the truth, she "admitted she had dropped Mr Godinez-Cortez off 'somewhere near the border'", prosecutors told the court.

"Sarah Branch added Mr Godinez-Cortez didn't come with her through the port of entry because he was 'Mexican'," Assistant US Attorney James Moore wrote in a court filing

When an officer checked Ms Branch's mobile phone, he discovered seven missed calls from Godinez-Cortez.

Prosecutors said Branch told border officials she had been in a relationship with Godinez-Cortez for four years and the couple was engaged.

Godinez-Cortez also entered a guilty plea on Tuesday to unlawfully re-entering the US and awaits sentencing.