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
Pretty cheap to unlock the telstra one through ebay code. I think I paid under $5 bucks for a code.