This is a good answer but does not focus on the OBVIOUS abuse of the system. If this happens more than statistically justified - there is something seriously off with morale, regardless how bonkers the rule is.

The OP has not indicated any abuse of the system. The OP said every single sick notice is accompanied by a certificate from a medical professional. That does not seem abusive at all. Neither of us would be qualified to question a medical professional's judgement, especially not since they have examined them in person while we are only reading hearsay over the internet.

@nvoigt This is Australia we're talking about, and speaking as an Australian, it's entirely possible to get a doctor to sign off on a "mental health day". Or, at least, it was, prior to the pandemic.

Self-evidently the whole point of the question is slackers cracking sickies.

I don’t see any mention of obvious abuse or fake sick days. If you do, please write your own answers, that’s why we can have more than one.

@TomTom: If an employer is difficult about sick leave being taken (staffing etc), good workers will try to not call in sick when they still think they might feel better by the time they need to start work. This causes people only calling in once they know they can't make it. That's a morale issue, but one of the employer's own doing, and not "obvious abuse".

@nick012000: If you are suggesting that "mental health days" are not correct or are given unfairly, this is a problem you have to tackle on a political level. Ot does not at all imply that every sickness or mental health days prescibed by a professional is fake.

@nvoigt "We have about 20 staff working and have at least 4 of them randomly messaging me on a Sunday night (or another weekday night)" - This is 20% sick leave, which is extremely high.

@Taemyr Do you suppose the OP gets those calls every day? Or might that be a spike?

To add: On every occasion I've had in the past couple years where I called in sick, typically I felt unwell towards the end of the previous day, went to bed feeling awful, and called in sick when I woke up. 24 hours notice is utterly pointless because with rare exception the illness wasn't even recognisable 24 hours prior. I might be able to get a GP's note, but in most cases it's a 24h bug anyway, so they wouldn't be able to recognise the symptoms by the time I felt well enough to go get one. Neither the notice-period, nor the doctor's note is meaningfully actionable in 99% of my experience.

[cont] So from an employer's point of view, I called in sick with zero notice (particularly since it's usually a monday for whatever perverse reason) and can't provide any medical backup of it. Fortunately my bosses have generally been supportive and trust that I'm not abusing the system.

@Taemyr there are a lot of symptoms which would have been ignored by most people in normal times which could be an indicator of CV. it shouldn't be a surprise that people are taking more sick days than they were last year

Your example is a strained ankle, but the point you are making even holds for many sicknesses - even with a common cold or stomach bug, it's common to notice the symptoms only after waking up in the morning.

I would add that with the current Covid situation, it SHOULD be like this specially when caring for children. Any symptoms (being relative to covid or not) should let the barer stay home at least 2 days.

@nvoigt - but, not noticing that Australia has "sickie culture" would be like not noticing that Australia has sunshine and kangaroos. It's just completely .. bizarre .. to discuss "managing sickies in Australia" without that elephant-in-the-room concept. It's whacky !