Business cabins on a long-haul route such as to New York are highly, highly sought-after, so it doesn’t surprise me that you’re having some real difficulties finding the availability. Reward business seats haven’t been phased out (velocity still offer them) however the airlines make them available based on the demand. During off-peak seasons when they’d be struggling to fill the plane you find these seats are easier to get, whereas during a peak period for travel such as around school holidays, Christmas, public holidays etc. they are tough to come by. The airlines know the demand is there and they will fill the plane anyway.
There a couple of ways you can still snag the business seats which you’re after. It’s risky, but waiting closer to the date it does happen that seats open up for booking if Virgin or Delta or whoever it is notice the flight isn’t selling how they hoped. I say it’s risky because there’s no guarantee they’ll open up and you may end up having to buy a full-fare ticket to get away at the last minute.
The other option would be to buy a full-fare premium economy or economy seat, and to then use your velocity points to request an upgrade. This would be an especially good option if you have some status, and seats by this route are still awesome value compared to what you would pay cash.
Flexibility is your best friend here as well. August you might find is the best month, just because it’s after the July school holidays, before the september ones but still really good, warm weather on the whole, so it’s generally a period of lower demand. Also August reward seats would only have been on sale for less than a month, so you’re not too fare behind the 8-ball. Consider travelling different routing than just the traditional Syd - Lax - NYC, because as a rule the most convenient, quickest, straightest route is the one most people want. Delta or Virgin America might have no seats going say LAX to NYC, but they may have availability LAX to Minneapolis or Salt Lake City or Atlanta or something and then onwards to New York as well. Again, far from ideal routing but that may well help you get what you need.
Hopefully you’re able to get what you need!