She suffered second-degree burns to the left side of her chest, shoulder and arm.
The Florence Smith nursing Facebook page posted graphic photos of the burns and said the nurse was writing notes in the nurses station when the boiling water was thrown at her, causing second-degree burns and "future scarring, not to mention emotional trauma".
"When questioned why, the patient shrugged their shoulders," the post said.
A DHB spokesman said earlier the nurse involved in that incident was doing well and now in a comfortable condition.
"We are carrying out a thorough investigation into what happened and the circumstances in the unit at that time and will continue to support the staff member with their recovery.
"Specialist Mental Health Services takes all incidents of violence seriously. No level of violence towards staff, patients, or anyone else is acceptable."
CANTERBURY DHB HEAD TO MEET STAFF OVER SAFETY CONCERNS:
Today's attack comes as the CDHB head, David Meates, is due to meet with mental health nurses at the facility tomorrow, to discuss safety concerns and violence against staff.
In a statement released late this afternoon, Meates said: "I am extremely concerned to hear another staff member has been assaulted at work in the adult mental health inpatient unit.
"We are doing all we can to ensure this nurse is well supported in their recovery.
"Whenever there is a serious incident such as this, a review will be carried out to understand what happened and what can be done to reduce the chance of a recurrence. That process is underway.''
He said no level of violence towards staff or patients was acceptable and was open to hearing ideas from staff to reduce the risk of violence in the workplace.
The latest incidents are not the first serious assaults at the hospital this year.
In March, a nurse was "beaten senseless", another's finger was bitten "to the bone" and a third nurse was hit on the jaw during an assault at the hospital.
The three nurses, from the mental health clinic, ended up in the emergency department after the assault by a patient in a drug-induced psychosis.
A health and safety review was put in place at Hillmorton Hospital in October 2017 after earlier reports of staff being bitten, hit, and injured.
Between January and October last year there were 11 ACC workplace injury claims for staff working in the assessment, treatment and rehabilitation unit.
Ministry of Health acting chief nursing officer Jane Bodkin said the safety and wellbeing of nurses and other health workers was extremely important and was the joint responsibility of individuals and the organisations they were employed by.
"We are very concerned about the rise internationally, and in New Zealand, of violence against health sector workers and welcome moves to work together to improve safe staffing and healthy workplaces."
Four people were also injured after a patient lit a fire at the hospital in December.