Hi,
We are a Melbourne based club, with a separate AMTRA group in NSW.
There is a hire place in Melbourne, called Garners, which includes off road bikes. Depending on the exact time that you are in Melbourne/Vic we may be able to host a ride. We are a club, rather than a tour operator, so our rides happen when members have time off work. There are some tour operators in Victoria, some of whom tie in with bike hire as well. Check out Mt Buller motorcycle adventures as one such operator.
Jan/Feb is a very hot time of year, not the best for trail riding as very dusty as well. But we have members who ride through the year.
For your brother in law to ride he will need a licence, although the learners permit course would be sufficient to get out legally. As a club we expect all riders to be licenced and have registered bikes, as that is required to ride on public roads and tracks in the forest areas.
Hope that gives you some ideas to make plans on.
Don't know how long you intend to stay, but Mel, Syd and Alice is a lot of travelling distance even flying between them.
Regards,
Mat.