are only resolved by supply

Eh, I don't agree. Supply is super important, and we definitely need to see a lot more of it. We need a doubling or tripling of the amount of government housing. We need zoning regulations to allow medium density at least in all cases. We need higher density in inner suburbs and near train stations.

But regulation protecting tenants is also useful. Vienna is probably the gold standard, with multiple different tiers of pricing regulation for different kinds of property and different needs. Other kinds of non-price regulation, like bans on no-grounds evictions, protections for tenants on quality and repairs, and rights to make minor modifications, are also important regulation. These go very well hand-in-hand with a levy like OP proposes, because the levy stops an owner from throwing up their hands and saying "I don't want the tenant to have these rights, so I just won't bother renting it out."

There's also just the simple fact that even with the very best of intentions, increasing supply is a years-long endeavour. Price controls can be done right now and take near-immediate effect. In a crisis, we can't afford to wait around for the long-term solution. We need something that works right now, in addition to addressing the root cause.

Yep. I bet my landlord is barely, if it all, breaking even on yield. Whereas I'm sitting on my arse investing in ASX: VAS getting 12%.

Why people go the lengths to get a loan, buy a flat, find a tenant, do all the maintenance and paperwork, all to get 0.1% yeild, and maybe an increase in value? I mean the last 30 years yes it would be wise, but you can't just say it'll be like that in the future.

Also, if I need the equity, it's liquid, unlike houses.

Because renting it out for a lot is better than tying up the market; at least it'll soak up some rich idiot.

Also it's not our money - it's coming off the new tax.

So, a common suggestion I've seen (and even made) in the past is to put a levy on un-occupied homes. But one problem with this is enforcement: how do you tell it's unoccupied?

Well, OP here has the solution. Flip the burden of proof around. Charge everyone, but if you can demonstrate that you have a tenant, you get that charge waived.

Then, OP adds further nuance to it. Instead of being a binary "rented/unoccupied", there are tiers. Start with unoccupied at maximum tax. Down to minimum (potentially zero or even negative tax) for below-market rent. It's just all described in terms of starting with an automatic levy of a certain amount, with rebates on that levy to get you to the desired rate.

Well, not quite.

The only subsidy would come from the taxes raised in the first place, so it would be revenue-neutral.

Raise taxes on every non-primary-residence, hand them back again if they're rented out.

My only proposal is to skew that handing-back to favour those that rent below the market rate, dragging the market down with them.

Primary sources, no - just a lot of people talking about it everywhere I go.

Certainly there seem to be vastly more dwellings than households - why do you think that is?

I have lived in apartments in Western Sydney and in Newcastle that had less than half the apartments in the block tenanted.... yet none advertised. I do not know the motivations of the owners. But it is quite common, in my experience. Perhaps they have no one pass through their doors for six months at a time or more for another reason, but I cannot think of one.

Actually no.