@Amanda58
Hello Amanda
For Russians visiting Australia they need a valid tourist visa. Some documentation is requested during the application but there is certainly no need for a tourist visa applicant to provide either evidence of where they will stay or supply receipts for hotel bookings.
One of the requirements to apply for the visa is that IF you are visiting a friend or relative in Australia, this person can act as a sponsor for the trip, and if that is the case then the sponsor should write a formal letter and give details about income and so on.
I'm taking a guess that this is what they might think they should include in their application.
It clearly doesn't apply in your case here, so what to do? I would explain that you cannot supply a letter, suggest that your potential guests should re-examine the visa documentation checklist more carefully, and perhaps they should consider hiring the services of an agency which specializes in visa support.
However there are several types of visa offered so there can be various nuances as well.
I have to say I've became quite interested in the process of visitors asking for advice on applying for a Russian visa as it is fairly common for me but I would say for the majority of hosts the best bet is to steer clear, keep out of giving visa advice and let it be the responsibility of the guest to navigate the process.