Won't that leave the coupler too low...?
I had remembered that there were buffers in NSW, but I'd forgotten that, in the diesel era, they joined them together!
Do be aware that 20 series couplers are plastic. The 40 series metal couplers are being replaced by 140 series couplers, but there is little sign of additional 150 series couplers, yet.
I think that your coupler will be low, and that you'd do well to, at least, try a #58 coupler, even if you have to file back the top of the trip pin (where it is staked). It may be that you will need a long coupler to clear the contraption between the buffers.
Kadee have a nifty chart that shows the availability of couplers. http://www.kadee.com/htmbord/coupler.htm