Are they?

Im not a TV guy, but I dont think they were ever commercially available here.
Doing a search on the National Library of Australia's "Trove" newspaper archive, I found a couple articles from 1954 where a contact was signed between Zenith & Rola for production of "phonevision" (coin-slot pay as you view) televisions to be manufactured in South Australia beginning in 1955.
After that, there are very few hits for "Zenith TV", apart from an occasional classified ad for cars with a Zenith radio, but even those are few and far between.
There doesn't appear to be any retail stores who advertised Zenith televisions, even into the 1970s & 80s, so I'd say they were not sold here.
I cant say I spent a lot of time reading the hits that did come up, so I dont know if the phonevision sets were eventually produced, or if they were produced under another brand name.
If you did happen to find a Zenith branded TV here, more than likely it would have been brought here by an American immigrant, although I doubt many would have bothered seeing as a step down transformer would have been required.

American TV sets operated on 525 interlaced lines, and the sound subcarrier was at 4.5MHz. And the TV channels were at different frequencies. And the power required was 120VAC, so with all these differences there'd be no point in importing American TV sets. And the market in Australia and New Zealand was less than 10% the American market, so there'd be little incentive for Zenith to design a TV set that could work in Australia and New Zealand. Heard that there were steep tarriffs anyway. Zenith could licence the name and logo to a local maker, but that's a marketing issue, if the Zenith brand name would mean anything to Aussies.

These issues didn't have an impact on AM radios, as there is really only one way to AM modulate audio onto a radio carrier, unlike TV where various parameters can be different. I have a couple of Aussie AM radios, and they are perfectly happy to receive American AM radio stations.

I am not sure if this was common elsewhere in the world though here the picture was transmitted in AM and the sound in FM.

Whilst a standards converter and stepdown transformer would probably suffice to get a US set going here it's a lot of work to do if foreign receivers don't become a collector's specialty.

These beautiful old sets are very nice though