Richard Royal had multiple fractures to both sides of his head and to the back of his skull, subdural bleeding and bruises on his face, she said.
Dr Hobbs said it took significant force to cause just one skull fracture.
There was also extensive bleeding inside the cranium (the part of the skull which holds the brain) and in the brain lining, she said.
"The whole area of the back of the baby's head was depressed with swelling over that, and there were also a number of lesions on his face, including on his left cheek."
Under questioning by Crown prosecutor Anna Pollett, Dr Hobbs said it was "likely that more than one significant force impact" caused the head injuries.
Dr Hobbs said the injuries were caused by a similar force to a high-speed car crash.
"Either the baby's head impacted on something or something impacted with the boy's head," she said
Defence lawyer Matthew Bates put it to Dr Hobbs that she could not say with any certainty the injuries could not have been the result of just one significant impact.
"Yes, but when you look at the injuries it would have been similar force to a high-speed crash and there was definitely a hard surface impact with the baby's head,"she said
Mr Bates also asked Dr Hobbs whether she had considered the bruising on the left cheek may have been the result of finger pressure from an adult's hand.
Dr Hobbs said she had not, as the pattern of bruising suggested what caused the lesions was entirely different than what caused the other injuries.
"I have never seen this type of bruising in accidental injury cases . . . " she said.
The defendant's principal lawyer Rob Stevens, who made a brief opening statement on Tuesday, urged the jury to keep an open mind and wait until they heard all the evidence.
Mr Stevens said "this was a tragic case".
In a "brief moment" the defendant reacted in a way that caused the baby's death, and he accepted responsibility for inflicting the fatal injuries.
"But he did not mean to kill Richard Royal," he said.
Mr Stevens said at the end of the trial the jury could find his client guilty of manslaughter but not guilty of murder.
The trial continues today.