The 50-year-old is understood to have suffered a serious head injury in the accident and was said to be placed in an induced coma.
Harris, who was involved in a major crash at the TT after coming off at Union Mills in 2007, was reported to have been thrown from his machine in a high-side on the exit of a hairpin turn.
An eyewitness said nobody else was involved in the accident.
In December, Harris said he was using motorcycle racing as a tool to rebuild his confidence and health in the wake of his accident at the TT in the 2007 Superstock race.
His wife, Fiona Kelsen, is believed to have flown to Christchurch to be with her husband. The couple have a daughter, Petra.
Read more at http://www.crash.net/rr/news/226298/1/former-tt-star-shaun-harris-critical-in-hospital.html#6W1DZ71sLZ5SejR8.99