Maybe not a “specialist” in Australia and NZ, but the Elizabeth Holmes Travel Agency is very well-regarded here in Seattle, and I believe they were (and maybe still are) recommended by RS.
https://elizabethholmes.com/australia-new-zealand/
But I will say that New Zealand in particular is very easy to plan on one’s own, using resources such as the New Zealand Facebook group above, and the very helpful people in the Tripadvisor New Zealand forum.
We did a combined trip to the two 10 years ago, using our Alaska Airlines and British Airways to get to Australia with Qantas, and then paid for inexpensive tickets on Emirates from Sydney to NZ and back to Melbourne. WE visited the South Island only, and spent 2 weeks there, focused on hiking. If you enjoy hiking, I highly recommend the Milford Track guided hike with Ultimate. The guides, lodging, food, and scenery were all top-notch. all we had to do was show up in Queensland on the appointed day, and they took care of the rest.
https://www.ultimatehikes.co.nz/multi-day-guided-walks/milford-track
We had been to Australia before, so did not need help with that. We just wanted to spend a few days enjoying Sydney, and then went to Melbourne after NZ for the Australian Open, which was a great experience. Highly recommended if you are a tennis fan and going in January.