I saw an ad for an HBO Max movie on Prime (i.e. available on HBO Max). That would be like a channel 10 program ad on channel 9!!
I had a look around and it seems that HBO Max is available through Prime in some regions as an additional subscription, but not here in Australia. I’m not using a VPN.
HBO Max has been available as an add-on for Prime Video since September last year here, so it makes sense they would be advertising.
https://au.variety.com/2025/tv/news/hbo-max-prime-video-add-on-in-australia-27192/
As far as advertising on direct competitors’ platforms, there seemed to be a lot of Stan ads getting around on Seven during the summer. I know Seven doesn’t have a premium streaming option/platform, but still it was notable.
Look, I saw an SBS ad on 10Play during Survivor. It’s quite comical how often they do this now
I’ve said previously about how the main commercials are a bit more open to promoting the ABC and SBS given their status as government-funded broadcasters (in the case of SBS, partially funded). For example, Nine openly promoted SBS’s coverage of Eurovision in their segments on Today with Delta after her announcement as the act - whether there was some kind of agreement, I’m not sure, but they have done similar things in the past when interviewing guests with ABC/SBS shows.
Having ads for a premium streaming service owned by a direct FTA rival is a new step though.
This discussion reminds me of this back in 2020. 
Yes you’re right. Sorry I wasn’t able to find it before on Prime itself, and Google Search just lead me to links saying it’s not available. But I found it now on Prime itself.
Since the 90’s there have been Foxtel ads on free-to-air TV, as pat TV was seen as different, compared to a rival commercial station. The same would be true for online streaming services.
As for the ABC & SBS, I have seen or heard ABC TV programs advertised on commercial radio, and ABC radio stations advertised on commercial TV.