A woman has been airlifted to hospital with serious head injuries after she crashed her quad bike into a cow, which then fell on top of her.

The 55-year-old was riding the vehicle on a property near Tallandoon, about 40 kilometres south-east of Wodonga, when she hit the animal about 11.30am today.

An Ambulance Victoria spokeswoman said the the bike flipped over and landed on the woman, after which the cow toppled over and landed on the bike.

Paramedics estimate that the cow and bike together weighed between 800 and 1000 kilograms.

The woman was able to free herself and was helped by another person to a farmhouse where an ambulance was called.

She was taken to a nearby hospital and this afternoon was airlifted to the Royal Melbourne Hospital, where she arrived about 4pm in a stable condition.

The condition of the cow is not known.