Since the media generally inconsistently report percentages I'm not sure if the stated 5-6% and 12% improvements are really in percentage terms or if they mean percentage points. If it's the latter, those would be very large effect sizes.
"Next week the Tax Office will go public with details of an extraordinary behind-the-scenes re-engineering of the way it interacts with the public......The ATO has been quietly trialling different ways of asking for money. It has set up a ''simulation centre'' in Brisbane to present pretend letters to real people and see how they react. It presents pretend web interfaces as well. Then, just as Google tests new search algorithms by randomly dishing up them up to some customers and not others, it posts new letters to 1000 of its randomly selected customers and old ones to the rest.