What are the dates? I would guess this has to be an end-on-end ticket? The DPS - SYD is likely what throws it off -- having flown to the region a few times over the past 18 months, I find that DL doesn't seem to permit any routings from US ---> Australia or v.v. via Asia -- which I'm guessing is where it's getting tripped up.

I have tried many many times to route SYD/MEL - ICN - XYZ (DTW / ATL / SEA) - RDU in an effort to get more miles when I've got time to come home on a more leisurely route and there is no way to get such a routing ticketed which is what makes me think a similar scenario is at play here.

So you need it make it into a multi-city so it treats DPS - SYD as a separate leg instead and presumably tap into a different set of fare rules

Try doing this as a multi-city, I can pull it up on Priceline and Virgin Australia (with DL flight numbers on all but DPS - SYD) doing the following:

NYC - DPS
DPS - SYD
SYD - NYC