You need to be registered for GST if you think you will exceed $75,000 in total sales during the tax year. And you will need to have an ABN as well.

As already mentioned I don't believe it matters if you sell off-shore or not. All sales count towards the $75K total.

I use the file management centre to download the Paid and Posted report each month and keep that by month in an Excel spreadsheet. It's then a very simple matter to sum up each month's sales and roll up to a year.

The above was some guidance given by my own tax agent.