Activate iPhone with any sim? I have bought an unlocked iPhone 5s (I believe it's model A1530) from the Apple Store Australia, can I use any sim to activate this phone?

Could I even use my partners and then put it back in his phone once it's activated?

I only want to use the phone over wifi for now.
If the device is unlocked you can borrow a sim to activate the device. One thing to keep in mind though is you may want to skip setting up iCloud until after you remove the borrowed sim card.

After you've ejected the sim go into Settings > Phone > My number. Either clear the number out (which will be the number of your borrowed sim) or type in your actual phone number. The reason being is iCloud (specifically, iMessage) will want to tie the phone number from the borrowed sim to your iMessage account. It could potentially end up with you receiving iMessages intended for the person you borrowed the sim from.