Smith (31), the All Black fullback and vice-captain, failed a concussion examination late in the first half of the opening test against the British and Irish Lions and did not return to play.
It was his third concussion of the season and there is concern over when, or if, Smith will be back on the field.
Leading sports medicine doctor David Gerrard said concussion was not simple and no two players experienced the same symptoms.
He said Smith displayed signs at the game he had picked up a concussion and the All Black management had taken the conservative line and taken him off the field.
``They would have erred on the side of conservatism and would have known he had had a couple of knocks already this year,'' he said.
``Obviously, there is no hard and fast rule as to when a player can and cannot stay in the field. Jerome Kaino went down later on and he passed the assessment and could have gone back on.''
Kaino was replaced by Ardie Savea in a tactical substitution and did not return to the field.
Gerrard said concussion was a very vague condition and, although plenty of work was being done, there was still a lot unknown about the condition.
``No two players will respond exactly the same way. It it not like a ruptured ligament or a cracked bone, where you can say with some confidence you are out for six weeks or three months.''
He could not advise what length of time Smith should spend off the field as players recovered at different pace.
He imagined All Black medical staff would take a cautious approach with Smith and a leading neurologist or neurosurgeon would be involved in the decision on when to get Smith back on the field.
Gerrard said the sideline concussion test gathered various information. There were such things as walking in a straight line, recall of numbers and testing short and long-term memory to determine the athlete's ability to get back on the field.
The threshold had deliberately been set very low, Gerrard said.
He said it was possible for players to be stunned but not be concussed.
``We have seen that - guys who have had to go off and have headaches and blurred vision who with 15 to 20 minutes could get back on and play.''
Players such as Richie McCaw and Josh Kronfeld had picked up concussions and returned to the game so that showed it could be done, Gerrard said, which was encouraging for Smith.
At a match two doctors met - the team doctor and the independent match doctor - and discussed the condition of the athlete. He said there had never been, in his 15 years of being a sideline doctor, a disagreement over whether a player can return to play.
Josh Kronfeld said he had failed two tests but passed the third one and returned to the field after a couple of weeks.
Kronfeld said he felt for Smith and it was an uncertain time for the All Black fullback.
Kronfeld had concussed himself earlier this year, when surfing, and said he was still feeling the effects of it.
