Ambulance services across the country will be at reduced capacity on Saturday during the second strike this week by St John workers.
Members of First Union and the Ambulance Association will take rolling strike action for 24 hours from 4am tomorrow, standing down for the first four hours of each shift.
This follows an identical action on Tuesday - the first time ever that ambulance workers had taken strike action.
Hato Hone St John deputy chief executive Dan Ohs said about 300 ambulance workers and call-centre staff took part in the strike earlier this week, and St John expected about 250 would be involved tomorrow.
"However, it's coming at a time when we have a higher workload volume - Saturday is one of the busiest days of the week for us - and so [there's] more risk to overwhelming available resources.
"Weekends are when people are involved in social and recreational activities, sports, travel and events, or DIY activities around the house that may lead to them getting injured."
St John was also attending 19 different events around the country on Saturday, which would further stretch resources.
The service had contingency plans with its "union partners", but would still only be running at "about 70 percent" of its usual capacity, Ohs said.
"That's why we're asking the public to leave ambulances for medical emergencies and serious injuries.
"For non-emergencies we would ask the public to seek advice from Healthline, an urgent care centre or pharmacy first."
Calls to 111 during Tuesday's strike were down 15 percent.
Hone St John had not identified any "patient harm" from the industrial action, and Ohs said it would be doing all it could to maintain a safe level of service this weekend.