This is the lowest cost Telstra 4G connection that I am aware of. $30 per month if you have a concession. The link is here. Only 2Gb data but enough for the mostly email and occasional web browser.

I like Samsung Mobiles, I usually check the Post office for specials. Currently I like the A30 at $329.  If you need more data to for example watch video plans start from $50 per month for 50Gb less $10 for concession holders.

There are other carriers and pre paid plans that cost less.