the 4wd crashed into a lake in melbourne 's outer west just before 4 pm .
there were four children and an adult inside the vehicle .
all the children are aged under seven years old .
three children have been confirmed dead by victoria police .
a fourth child remains in a serious condition at hospital .
a woman has been transported to hospital and is under police guard .
police say they are yet to determine how the car went into the lake .
` police do not believe anyone is still in the lake , ' victoria police said .