What are the transaction fees?

There are no monthly fees at all - so that's a great start.

And here's a breakdown of The Nimble Visa Prepaid Card fees:

Nimble Card Fees (Payable to the debit card provider)
Monthly maintenance fee FREE
Online balance enquiry FREE
Cardholder service desk FREE
Fraud monitoring FREE
POS purchase or decline FREE
ATM withdrawal (Australia) ATM owner's fee
Card load (pay anyone from bank) $1.99
Card load (pay anyone out) FREE
Card load (BPAY®) $2.79
PIN reveal FREE
Foreign exchange fee (overseas transactions) 2.99%