The emergency department had regularly had more patients than beds in the past few days and weeks, and urgent care clinics were also overrun, despite falling Covid numbers.
In a newsletter to staff, Te Whatu Ora Canterbury's director Peter Bramley said all areas were impacted - from GPs to surgical wards.
"As we welcomed spring, there was hope that the worst was behind us and we could catch our breath before the next Covid-19 wave appears," he said.
"Unfortunately, things haven't turned out as anticipated and we've seen unprecedented high demand for acute care throughout our health system."
Last week was so busy, urgent care clinics had to shorten their hours and the hospital urged people to keep the emergency department for genuinely urgent cases, telling them to treat sprains at home with a bag of frozen peas.
Bramley said staff illnesses were exacerbating the problem, with the school holidays adding to pressure.
At 3pm yesterday the hospital was 92 percent full.
The Nurses Organisation was encouraging all 35,000 of its members working in hospitals nationwide to turn down extra shifts this week.
The move was a protest at Te Whatu Ora/Health NZ's refusal to negotiate new compensation for staff working extra hours, now a bonus that existed for extra work during winter has finished.
Christchurch Hospital said it had not seen an obvious impact on the first day.
Te Whatu Ora Waitaha Canterbury's executive director of nursing Becky Hickmott said they would manage the need for any additional staff by moving nurses from its existing pools to the areas they were needed most if necessary.
