An investigation has been launched by Nelson's health board after a patient from its mental health unit got into a sixth-floor diagnostic area, broke a window and jumped out, landing two floors below.

The inquiry is being classed as a sentinel event investigation, which follows any event where a significant failure of the hospital's care is possible.

The Nelson Marlborough District Health Board is not yet revealing how the man got into a Nelson Hospital stairwell in the sixth-floor diagnostic services area of the Percy Brunette Building - which contains no wards - about 9am yesterday.

The man, aged in his late 30s, smashed a window with a fire extinguisher before jumping out and landing on a cooling tower two floors below.

He was in the hospital's intensive care unit in a stable but improving condition this morning.

Health board incident controller Pat Davidsen said police had completed a scene investigation.

He could not say how long the sentinel event investigation would take but said it would be thorough.

Senior Constable Phil Wylie of Nelson police said an officer attended and, on advice from a physician, decided not to take any action.

Updated from http://www.stuff.co.nz/4744387a20475.html