For those looking for a cheap 4G modem, Australia Post has a couple on special until 23 December.
They have the Optus Huawei E5577 for $29 with 14GB of data, and the Telstra 4GX ZTE MF910V Wi-Fi Modem with 12GB of data for $29.50 (both normally $59).
They're CAT4, meaning they don't have carrier aggregation. They can still manage up to 150/50 speeds though.
I would presume both are locked to their respective carriers.
Specs:
https://offer.optus.com.au/shop/prepaid-device/wifi-e5577?SI…
http://www.ztemobiles.com.au/MF910V.htm
I did this and use the Huawei b525. Range is good but I sometimes suffer drop outs, hard to tell if it's the router it optus.