The proposed changes include providing more information on certificates and detailing which duties a patient is fit to perform.
Ms Duncan said any absenteeism was costly for businesses.
"A move to stating what a patient 'is fit to do', rather than a blanket 'sick' could be helpful, avoiding the need to divulge confidential information.
"Good employers work with genuine sick staff to manage through periods of genuine prolonged or regular illness.
"Those who are able to use the current system to avoid regular attendance at work do a disservice to both their employer and workmates," she said.
New Zealand Medical Association chair Dr Mark Peterson supported a shift toward a "workability note", where doctors' detail on certificates which tasks a sick or injured employee can perform.
This system already operated in Britain, and local doctors were increasingly taking up the practice, he said.
For example, if someone employed as a check-out operator had an ankle operation, they "clearly can't work as a check-out operator because it involves standing all day".
"In that situation, the note would say 'this person's fit for work but only in a sedentary role'," Dr Peterson said.
A previous Treasury report estimated lost productivity caused by ill health of workers cost between $1.44 billion-$1.76 billion each year.
Dr Peterson advised any employers who were suspicious to speak to their employee.
"We certainly won't disclose the reason for the off-work certificate without patient consent."
Doctors could be approached once patients gave their permission, he said.
Abuse of the system was also unlikely - doctors only issued certificates when they had evidence someone was sick or injured.
Despite this, employment lawyer Max Whitehead said some doctors issued certificates when they weren't necessarily needed. "Rather than challenging a patient's honesty, doctors declare them as sick," he said.
Sick leave
Workers are entitled to five days' paid sick leave after six months.
Following this, a worker is entitled to five days' sick leave for every 12 months.
Sick leave can be taken if a worker is unwell or if someone who is dependant on them is sick (children, spouse).
An employer can ask for proof of sickness or injury as soon as sick leave is taken.
If a medical certificate is requested within the first three days of sick leave being taken, the employer must pay for the doctor's visit.
The employer is forbidden to require the worker to visit a certain doctor.
Source: Ministry of Business, Innovation and Employment
