howdee – I assume that you’re referring to common property access keys, where as the landlord you are indeed required to provide one to each tenant shown on the Residential Tenancy Agreement (RTA) for your unit, and may charge / pass on any legitimate fee to other occupants who also require one.
As for the security deposit, it’s quite common for Owners Corporations to resolve to use that means to ensure that occupants attach some ‘value’ to their means of access to the property so as to avoid site security being compromised when access devices become like backsides (i.e. where everybody has one), and of course to cover its cost of administering the system.
So in that context I don’t regard a $100 security deposit as unreasonable, and it’s only in the event that you choose not to return common property key/s that are in excess of your needs at the end of a RTA or if a key was lost would that deposit not be refunded.
In the overall scheme of things a few hundred dollars is a deductible drop in the rental ocean, but if you object you can always raise your concerns by way of a Motion on the Agenda of your next Annual General Meeting.