Gday Arief, welcome I would suggest around November because of the added benefit of the wildflower season.I cant give any recommendations on farmstays but theres a healthy selection to choose from , try here
http://www.westernaustralia.com/en/ . Wheatbelt region of WA would present good conditions , although it would be still cold during the nights . Another good locale would be some farmstays around the Stirling ranges about 3hrs southwest of Perth , some good daytrip opportunites .
http://members.westnet.com.au/web/astroventures/
http://www.gdc.asn.au/seestars.php
http://www.stargate-observatory.com.au/