Importing National Australia Bank (NAB) account transactions into GnuCash is reasonably straight-forward. It saves considerable time when entering more than 10 transactions. The process took some time to figure out so I've documented the relevant steps here.
1. Export your transactions:
Select "Quicken version 7 or above / Microsoft Money (QIF-local date)". Hit OK and save the file TransactionHistory.qif
2. Verify the QIF file.
Open the file TransactionHistory.qif in a text editor. Each account transaction takes a few lines and ends with ^. This page describes the QIF format. Check the top-most transactions to make sure the "P" lines are complete. Either fill the payee information manually, or delete these transactions.
3. Import the QIF file.
Click through the wizard, at "QIF import" it will take some time while it performs fuzzy-matching of the new transactions with your existing ones.
At the "Select possible duplicates" wizard dialogue carefully check through each potential match GnuCash finds. Tick the "Match?" box for each matching transaction. Ticked transactions wont be imported. I've never had an incorrect match suggestion at this step.
Each NAB transaction can be matched with the appropriate expense account in the wizard. I tend to skip this step and fix the account matches in the ledger later. Unmatched transactions appear in the account "Unspecified".