When I inserted my TPG micro SIM card into the iPad, I expected it to work right-away when I turned "Cellular" to ON in the iPad "Settings". However, it was not quite so simple.
First, I had to go to my TPG account, click on Mobile Service Control Panel for my designated phone, and ensure that "GPRS (data)" has been enabled.
Next, on the iPad cellular settings, I had to enter the APN, as well as the correct ID and password for my TPG account. APN stands for Access Point Name. For the different service providers in Australia, refer to: http://whirlpool.net.au/wiki/mobiles_apn. The APN for TPG is "internet".
(Disclaimer: I do not know if the above is repeatable. I got to this point after I successfully unlocked my Pocket Wifi E585, which had been previously locked to Vodafone. In the process of getting the Pocket Wifi to work, I also managed to get the iPad to work using the same micro SIM card.)