There are grave fears for a Queensland mother after her two-year-old daughter was found wandering the streets of Mexico alone. 

A search is underway for 32-year-old Tahnee Shanks, who hasn't been heard from since Saturday. 

Her family believes she was on the verge of returning home to Australia after the breakdown of her relationship with her partner.

The pair was reportedly heading on a trip to Tulum before the toddler was found near the Chapel of San Archangel in Cancun, 130km away.

Ms Shanks' brother, Dan, said his sister was an experienced traveller and was concerned she hadn't been posting on social media or in touch with the family over FaceTime.

"It's just horrifying. I've had to bury a brother three years ago and now we've got a sister... She was meant to be here in a month's time and we were going to release my brother's ashes at the old camping place we used to go as kids on Stradbroke Island."

Members of the family, from the Whitsundays, were flying to Mexico to try to get Ms Shanks' daughter back to Australia and help search for the missing mother.

The girl's father is also reportedly missing.

Mexican police have issued a missing persons alert for the missing Australian, and Australian authorities are believed to be communicating with them

A woman who lives in the area going by the name Teejay Casado Crawford posted in several Mexican expat groups looking for help.

"They were travelling Cancun with their 2yo child from Merida," she wrote, on Tuesday (Wednesday AEST).

"Last night the daughter was found alone walking street barefoot in Cancun.

"The parents are no where to be found. We are very concerned, the mum is my friend and she would never just leave her daughter"