"This seems to me to be a way of putting more leverage on workers to get them back to work when they may not be physically or mentally agile enough to come back," he said.
"It also rings danger bells for me about doctor-patient confidentially. This would appear to open the door to employers expecting more and more detail about why a worker is unwell."
New Zealand Medical Association chair Dr Mark Peterson supported a shift towards a "workability note", where doctors detail on certificates which tasks a sick or injured employee can perform.
This system already operated in the United Kingdom and local doctors were increasingly taking up the practice, Dr Peterson 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 due to ill health of workers cost between $1.44 billion and $1.76 billion each year.
Dr Peterson advised any employers who were suspicious about sick leave 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, Dr Peterson said.
"We actually know work is good for people's psychological health."
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.
"This means the doctor gains financially and the powerless employer has to lose a day's work and pay a day's wages," Mr Whitehead said.
The Council of Trade Unions dismissed the claim, saying any suggestion of fake certificates was "an absolute outrage".