Depends what you want to see I think.
Personally I found Bali very disappointing for diving - the wreck at Tulamben is meant to be a highlight, but literally every time I've been there, I've been lucky if the viz was 3-5 meters at best. August is mola mola season though, so if you dive at Crystal Bay, you may see one.
Philippines is good for macro/critter diving, so if that's your thing, then I'd keep it open as an option.
Malaysia is fantastic, but it really depends on where you go. Borneo is great - Layang Layang or Sipadan would be the best options. I'd maybe avoid Layang Layang at the moment due to geography - it's in the South China Sea, and there's a lot of political/military posturing going on there right now with China that I'd be actively avoiding. Sipadan is mindblowing, but the permit system there is a game. In a 4-5 night trip, you will probably only get a Sipadan permit one day. Not to say that there's not other good diving around there on the non-permit days, but it's a gamble.
PNG is amazing, and you'll get more cultural exposure there too.
Another option if you're flying ex-Sydney would be Fiji or Tonga. Fiji's a bit of fun. And at that time of year, Tonga has humpbacks
The GBR has lots of colourful fish, good coral, and if you're doing the 4 day trip to Osprey, the sharks and rays out in the Coral Sea are a highlight. And August is kind of the back end of minke whale season, so you may be lucky
Really kind of depends what sort of thing you want to see.