Tan Tock Seng Hospital, Khoo Teck Puat Hospital and Yishun Community Hospital warned patients that they could experience longer waiting times because of the disruption.
Woodlands Hospital said in an update at 1.30pm that its call centre lines had been restored.
NHG Health advised patients and members of the public to visit its websites or use the HealthHub and NHG Health apps for assistance.
CNA’s calls to NHG Health’s main contact centre hotline at 10am initially failed to connect.
Calls to Tan Tock Seng Hospital and Khoo Teck Puat Hospital also failed to connect, although a call to the Institute of Mental Health went through successfully.
At the National Skin Centre at about 11am, staff told CNA that its payment system was down.
Patients at its clinics and pharmacy were unable to pay for consultations or medication and were directed to make payment through the NHG Health app instead.
NHG Health told CNA on Tuesday evening after the disruption that its patient care was not compromised during the incident.
A spokesperson said clinical services, including emergency care, inpatient care, surgeries, medical tests and scans, continued with minimal disruption as NHG institutions immediately activated their established “downtime procedures”.
“Medication delivery service also remained available through our pharmacies,” they said.
According to the spokesperson, additional staff and volunteers stepped forward to support ground operations and assisted patients with registration and payment at the institutions, which eased the disruption’s impact.
“Patients were also able to access selected services online via HealthHub, the NHG Health app, and AXS and could further refer to our websites for assistance and live chats,” they said.