If you don't need fast, then 2talk slow speed 1Mbps unlimited 4G for $15 +GST per month.
Another option is Skinny basic plan, then add a 24hr unlimited booster.
NB: the 2talk option is great for EFTPOS or other IOT stuff that doesn't need a fast connection.
You can get a Draytek router with built in 4G for Auto failover too.