I used phones from US/Japan on Optus, who is who I had, but never Telstra. Here’s the important part, the US iPhone SE supports these LTE bands:
1 (2100 MHz)
2 (1900 MHz)
3 (1800 MHz)
4 (AWS)
5 (850 MHz)
8 (900 MHz)
12 (700 MHz)
13 (700c MHz)
17 (700b MHz)
18 (800 MHz)
19 (800 MHz)
20 (800 DD)
25 (1900 MHz)
26 (800 MHz)
29 (700 de MHz)
I believe the 700MHz range is the one you need for Australia. Which it has:
Australia is currently using these LTE bands
2100Mhz (B1) – Telstra (a handful of sites), Optus (Tasmania), Vodafone
1800Mhz (B3) – Telstra, Optus, Vodafone
850Mhz (B5) – Vodafone
2600Mhz (B7) – Telstra, Optus
900Mhz (B8) – Telstra (a handful of sites, utilises spectrum previously used by 2G)
700Mhz (B28) – Telstra, Optus
2300Mhz (B40) – Optus (Vivid wireless spectrum)