It's a good idea to have a medical certificate policy for your workplace so that all staff are clear on your process for when they are sick and you need proof.
Employers should read the medical certificates carefully - some give an employee's opinion on whether they are fit for work, rather than a Doctor's clinical observation or examination. Employers can reject certificates that they have concerns about - and it appears that some Doctors are using computer-generated templates that may not actually be giving employers information they can rely on to support an employee's time away from work.
Here's the link to the full Medical Council statement. http://www.mcnz.org.nz/assets/News-and-Publications/Statements/Medical-certification.pdf