It wouldn’t bother me if they were legitimate tenants... it’s just that they’re not.You may not like it, but it is allowed. I have a client who pays rent once a year. The whole year at once. It is a legit rent payment of the correct amount. It is his choice to do that. And the landlord/owner loves it.
The ATO does not check for monthly, fortnightly, or weekly payments, or care. They are only interested in the annual records.
I also suspect that for some years no rent was paid & MC was just paying the mortgage yet she was also taking advantage of it tax wise.
Sorry for not being clear, but it seems suss to me & I’m trying to understand it.