Hi Dave,
Verizon is somewhat different from normal GSM/HSPA/LTE carriers in the respect that it still offers some of it's service over CDMA EVDOx1 A/B. The CDMA protocol is not usually delivered using a sim card only, and there are carrier settings in the iPad which are for this purpose.
I do not believe the iPad is locked to Telstra as 4th generation iPads are all unlocked by default. You need to give the MEID and IMEI number to Verizon. When they have been added to Verizon's network, then insert the Verizon sim card and completely factory reset the device. You will lose everything. When the device reboots, it should come up onto Verizon. If you restore, you may lose the Verizon carrier settings.
The 4th generation A1460 iPad seems to be a dual mode device between CDMA and non CDMA carriers. And the mode seems to be set upon the first encounter with a sim card after a complete reset.
To confirm the device is not locked, pop a T-Mobile or AT&T sim card into it. If it is locked, you will get a message stating that it is carrier locked. If not, you should see 3G service on either of those networks.
I would strongly recommend finding a small techie independent dealer who can probably help with this more than the staff in Verizon. Verizon is a very controlled network and BYOD is not massively supported at the best of times. They are unlikely to have experience of this and their text book response would be that they do not support it.
I'm also unsure as to whether or not Verizon offers LTE data on Prepaid plans so if it is a prepaid plan, you would only get 3G (CDMA) which after using Telstra would not impress you much speed-wise.
What may be an easier solution although not as desirable would be putting a prepaid T-Mobile sim card in. They now support DC-HSPA on 1900Mhz in many markets and it is nearly as fast as Verizon's LTE and quite cheap too. (With this T-Mobile solution, no reset would be required).
Good luck.
J.