Hi Todd. 

I also have a PK loop antenna, purchased at a hamfest about 5 years ago, I find it very handy for listening to 675kHz ABC Corwa in the mornings. 

I can confirm VOA Thailand does get to Melbourne, after my last post I tuned in at 10.44pm Sat night and heard Music, and another station with talk. The music station came up above the talk and I clearly heard the VOA ident. This was with the aid of the PK loop. 
Afterwards I tried again without the loop & VOA came right up with the ident clearly heard again. I was lucky the ident was announced on 2 occasions while the station came in strong. The radio used was the Digitech AR-1780 which is my main dx unit atm.

918 kHz sounds like one I could try too, but there's a local station here on 927 kHz who's spatter will be a pest. 
Also we have a 50kw station 3RN on 621 kHz so 612 kHz is difficult to work with. 

This afternoon, about 3.30pm  some residual E layer skywave was taking place, 900kHz 2LT Lithgow was clearly heard, as was 1053 kHz 2CA Canberra. Also the usual ones near the top of the band. 1494kHz 2AY Albury, 1521kHz 2QN  Deniliquin and 1566kHz 3NE.

Regards. Dan.