Number one, find out whether they want to pay an extra $15 per week. If no, don't do anything.
If yes, find out whether you can mutually tear up the lease (your state will have someone who looks after this, in Qld it is the Residential Tenancies Association - not sure about Vic) and sign a new one at the new rate, and include in the lease that air-con will be installed within xxx weeks. You can possibly put in that rent is $xxx per week until air-con is installed when it will rise to $xxx.
Your tenancy association should be able to tell you whether you can do this, but in my opinion, you are releasing them from their lease and signing a new one - but check it out.
If it was me, this is the path I would take, but because we generally sign six month leases, I would wait until closer to the new lease to install the unit and then sign them (or someone else) up at the higher rate.
I don't do things like this because the tenant wants air-con, but if I can get $15 per week extra, the air-con is paid for within two years and I have a much more attractive house to let for the next tenants.
Wylie