A hospital spokesman confirmed that the victim is a nurse and that "extra surveillance" has been put on in the wake of the attack.
The offender is described as slim with dark hair, under 160cm tall, and aged in his 30s or 40s.
He was wearing a baggy grey hoodie, dark trousers and a beige Halloween-type mask, and was carrying what appeared to be a small dark-coloured firearm.
"At this stage, this is being treated as an isolated incident and officers are focused on catching the man responsible," senior sergeant Vaughn Lapslie said.
Police would like to hear from anyone who may have been in the area at the time as well as anyone who recognises the offender's description, or who has any information about this incident.
Please contact Christchurch Police on 03 363 7400, quoting reference number P029029690.
If you prefer to remain anonymous, you can leave information with Crimestoppers on 0800 555 111.
