Like many users of NBN's long term satellite service we're discovering it is rather flakey. The satellite link itself is pretty solid, but the NBN supplied routing upstream of the ISPs has frequent irregular outages, leaving end users offline for periods ranging from minutes to hours, a situation which has been ongoing for months.
The symptoms when things go sour is that the modem is still online: it has a happy blue ring on it, and usually you can get DHCP from your ISP. Since the DHCP service is on the far side of the satellite link, the satellite itself is exonerated, as is the ISP. However, any IP traffic sent to the public network never receives any responses. This state can persist for some time; eventually whatever is confused in NBN's network becomes unconfused (or gets reset).
One thing we have found is that even after NBNCo gets unconfused, the network remains unusable until you renew your DHCP lease. And immediately you do this, the network becomes usable. (If NBN have their side fixed; there will be a period where no DHCP activity does any good, and DHCP queries may even receive no response.)
Therefore, when your NBN goes away, try renewing your router's DHCP lease. For us, this can lead to immediate renewal of service.
Until NBN sort themselves out we've told our router to keep leases for only 15 minutes, so that it issues DHCP renewals regularly and hopefully will bring our service live again automatically.
We're inferring from this that somehow the DHCP action triggers some form of ISP or NBN routing action because until the DHCP renewal happens not route is in place. Also, when NBN are still confused, doing a DHCP renewal can lead to successful connectivity for something like 10 seconds. This suggests to me that through some screwup the NBN-side state that routes to our service is in conflict with something, possibly another customer's service; after the DHCP our route is in place, and then it is quickly trashed.
I am posting this in the hope that other NBN satellite users will find the workaround useful.
Footnote: the advice from NBN or an ISP tends to be "turn off your modem for 2 minutes, wait for it to reconnect, then restart your computer". Restarting one's computer or router causes it to do a DHCP request on startup. In our experience of NBN's current outages, the only step in this process which achieves anything is the DHCP request. Everything else is just witchcraft (well, "reset all the client's systems to clear any state").
