You have three options. Either recharge early on the UNLTD offer, but a $10 Add On which includes 500MB of data, or once consumed, you won't have Internet access until you recharge again (except care.boost.com.au)
CORY | I do not work for Boost or Telstra.
Boost Care: 125 8881 or 1800 026 678 . Or Live Chat 24x7: http://livechat.boost.com.au