I have no idea why they would have you turn off your firewall to configure your account. Though, I am not familiar with Big Pond. Is that a Satellite provider?
You mention "both of your firewalls." You should only have the OneCare firewall active. Yes, you can turn it off by going to Change Settings in OneCare, but your status will turn to red and it is not a state that I would recommend connecting to the Internet in as you will literally be without any protection.
If this is the plan you will be using, http://my.bigpond.com/internetplans/broadband/wireless/overview/default.jsp, there is no reason at all to turn off your OneCare firewall as this is simply a cellular connection.
However, if they supply connection software that includes a firewall, there will be a conflict.
Are you going to be using their USB based "modem?" If so, there could be an issue with configuring it to work with the firewall, though I can't say for sure. My recommendation is to contact OneCare support, who should have experience with other customers in AU using this ISP and service.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve