They would have to register and they would have to have been away from Canada for less than five years. That's the current status of the law. If they've been away for more than five years or haven't resumed residence in Canada, they can no longer vote. I say that with a caveat, because the matter is before the courts. The Supreme Court will be hearing it next winter, in February or something like that.
On October 4th, 2016. See this statement in context.