20-50 years ago then absolutely he would have been gone.
But times have changed and people are more relaxed about this sort of stuff. It hasn't really done Bill Clinton much harm in the long run. In some quarters it improved his popularity
It will really depend on the exact circumstances but, in reality, this issue has been played out in the media ever since the incident at the Wine Centre and it doesn't seem to have put a dent in the opinion polls so far. They would have assumed she would eventually sell her story so they would be prepared for it.
This is all on the assumption that Rann himself doesn't choose to resign. I just can't see that he will.