Okay, as above, you *can't* run both ADSL modems @ the same time.
your username & pwd should be in only one modem, & it should have it's sync light solid/ON @ all times.
Call BigPond, if there is no sync light & report that, as a fault.
( they will send a tech out, to fix any faults)
connect *only one* PC via a ethernet cable (usually the yellow one that came with that modem),
& check that you can connect to the 'net - if not call BigPond, as above.
if you can. - then you will have to use a 4port/wireless router, to give the 'net axcess to the 2nd PC,
either via a long! ethernet cable or via wireless. - you can get both of these, from harveynorman, if you don't have one.