I think the policy is unlikely to change for US citizens in either country anytime soon. There is a long and strong tradition of young Aus/NZers going overseas on working holidays so reciprocity is quite a big factor in why the policy is there in the first place. Plus US citizens tend to return to their home country and don't generally get in serious trouble offshore, so they're not really seen as a big problem in immigration terms. Your age will start to be more of a problem (30 is the cutoff), though, so don't leave it too long.
If you like outdoor work, consider seasonal viticulture work (horticulture as well). The grape harvest is usually March through May, ish. Horticulture work tends to be a longer season by a couple of months. It's long days but it often comes with accommodation and food as well as pay, and usually a good, lively bunch of people to work with (and a source of ideas on other work). If you do that seasonal work for a recognized employer, you can also extend your WHV by whatever time you work on that. See this page for a list of NZ Immigration certified recognized employers
Recognised Seasonal Employer (RSE) List
The main areas for winegrowing are Marlborough (Blenheim), Hawke's Bay (Napier/Hastings), and Central Otago (Queenstown), but there are also vineyards around Auckland and the Bay of Plenty. They all have generally hot, dry summers, cool winters. If you decide you like that type of work, you could look to study viticulture in NZ and that can definitely be a path to residence. And a path to jobs in many parts of the world.
In winter, the ski resorts can be a good source of temporary work, and not just for ski instructors. Google New Zealand, jobs, ski resorts and a bunch of sites with listings will come up. They are all looking from about now till the end of Feb/March to fill positions starting June/July through till end of October, roughly. Again, accommodation often comes with it (but not always).
You may come across the very occasional "bloody Americans" comment, but generally people will be friendly and interested.
I don't know too much about work in Australia, but I do know that NZers often go to work in the mining towns in the outback, in central and northwestern Australia. It's hot, dirty, nasty work, too many men, not enough women, lots of alcohol - but the pay is phenomenal.
Good luck, I'm sure you'll have a great time wherever you end up.