[defrost]

The rationale, such as it is, is that BoM serves current and historic weather parameters for various parts of Australia and hasn't seen any need to ensure that be delivered in a secure manner to individual web users.

There might be some hypothetical scenario from faking weather data and injecting it to fool the casual user but that seemingly hasn't come up in practice and so they don't fuss about it.

On the flip side, for those interested in RAW downloads from MODIS and other sats relevant to the weather, to ground station raw data transfers, to modelled predictions under various assumptions for commercial | military | government use etc ... BOM has secure login and bulk data transfer protocols, and has had those for at least 30+ years (morphing with time).

[samjmck]

I think the more likely scenario for a MITM attack is to insert malicious scripts or links into the web page, not to fake weather data.