The men appeared to be in a hurry and were looking to buy a map, the witness said.

At 10.30am the next day, a Chevrolet van identical to the one being driven by the Australian pair was found burnt out in farmland at Navolato, about 200 kilometres south of the convenience store where the witness reported seeing the men.

Inside the van were two bodies that were so badly burnt that Mexican authorities were not able to identify them immediately.

A bicycle was found strapped to the back of the burnt van, similar to the one Mr Lucas and Mr Coleman had taken on their trip.

The bodies have now been taken to Mexico's Forensic Medical Service for identification.

But the families and partners of Mr Coleman and Mr Lucas, both 33, from Golden Bay in Perth's south, fear the worst.

Andrea Gomez, Mr Coleman's girlfriend, said she could not help but would partly blame herself if the pair had travelled through an area notorious for its drug cartels at night.

Mr Coleman and Mr Lucas were scheduled to travel south from Los Mochis to join Ms Gomez in the city of Guadalajara on November 21.

The location where the burnt-out van was discovered was on the route the men would have taken on their road trip south to Guadalajara.

Navolato is in the state of Sinaloa in north-western Mexico, and is home to the powerful and violent Sinaloa drug cartel, thought to be responsible for as much as half of the narcotics smuggled into the US from Mexico each year.

"They hurried to get here and I think they travelled by night," Ms Gomez told the Associated Press.

"I had told them of insecurity, but never told them of the horrible things that could happen. I feel a little responsible for that."

She said a person had contacted her to report seeing the men trying to buy a map the night before the van was found.

Ms Gomez and Mr Lucas' girlfriend, Josie Cox, were travelling to Culiacan, the capital of Sinaloa, to try to confirm if the bodies were those of their loved ones.

Ms Gomez said she was taking one of her boyfriend's dreadlocks that he had previously given her to be tested for DNA, while Ms Cox was taking Mr Lucas' dental records.

"The van matches ... but we have nothing confirmed yet, so Josie arrives today to Mexico from Canada, and early Monday morning we will go to Culiacan to see the bodies and testing of DNA, " she told AP.

Mr Lucas and Mr Coleman had been living and working in Edmonton in Canada before they embarked on their trip to Mexico.

They drove south through the US, and ended up in La Paz, Mexico, the last time their family and friends heard from them.

The pair are believed to have caught a ferry across the Gulf of California with their van on November 20, and disembarked at Topolobampo at 10pm.

Topolobampa is about 25 kilometres from Los Mochis, where the witness reported seeing the men attempting to buy a map later that night.

On Sunday evening, Mr Coleman's mother shared a link to an online fundraising page set up to fly the Lucas and Coleman families to Mexico.

On the post she said: "Our sons life has left us, but they will be with us forever in our hearts ... "

Another family member said the money being raised would help the families "bring our boys home".

The Australian Department of Foreign Affairs and Trade said the "families and partners are aware the van in which they were travelling has been located by Mexican authorities and that a tragic event has occurred".

"The families hold deep fears for the safety of their sons but stress that they are still waiting for details to be confirmed," the statement said.

"Members of Dean and Adam's families will be travelling to Mexico in the coming days and working with Australian consular officials and Mexican authorities. We ask for privacy during this enormously difficult time."