A Chinese mother, blocked from entering Australia due to the coronavirus travel ban, is losing hope of being able to see her only son before doctors in Melbourne turn off his life support.

Xiao Li, 22, who has been in the Royal Melbourne Hospital in a critical condition since his car collided with a truck on January 27, was declared brain dead on Monday.

Xiao Li came to Australia last May and worked on farms in Queensland and East Gippsland, Victoria.

After hearing of her son's crash, Xing Lan Ren scrambled to apply for an Australian visa by January 29.

Ms Ren fast-tracked her visa application at a cost of $1145, expecting to hear back within two business days.