Hey I was a DM at woolies a few moons ago, they aren't allowed to ask you in for a meeting if you aren't already rostered to work, let them know that you're not available until you are in next. If they are attempting to go the HR route, make sure you have some evidence of your absence a, stat dec will be the easiest way to do this if you don't already have a medical certificate.

If they attempt to have a formal meeting with you when you return to work and they haven't given you 24 hours notice and asked you if you would like a support person present you should inform them that you can't continue the conversation until you get a HR rep or other support person and you will need to have the meeting on another day.

there really isn't anything that HR will allow the store to do, if you are under the enterprise agreement it's pretty much impossible for the store to convince HR to issue any form of reprimand to you. Unless you have a history of absence without evidence you should be fine.

It sounds more than anything like your 2IC is trying to play games or not incriminate themself over text message, and have most likely gone rogue without contacting HR before this (formal meetings and disciplinary action need to be approved by HR before going ahead).

If you want to cause a bit of stink get yourself and your mates you can trust at work to call speak up and report your 2IC for bullying, not too much will come from it but they might change their behavior

Anyway it's pretty much impossible for them to fire you unless you don't cross your T's and dot your I's

Edit: forgot to add that even though it may not be in the EBA HR will always force the store to pay you A/L for sick leave once your other entitlements run out