So what are the factors that have made a difference over the past year?
A third of ACC's extra $1 billion in profit came from its fund management operation.
ACC's investment income was $1.75 billion, up $340 million on the year before and representing a return of 12.6 per cent on the corporation's reserves.
While ACC's fund managers are highly regarded and usually outperform their benchmarks, investment returns are obviously hostage to what is happening in financial markets. The period since balance date will not have been as rewarding.
ACC's reserves increased from $13.1 billion to $16.6 billion in the course of the last financial year.
But that still leaves them $6.7 billion short of the level needed to fully fund ACC's liabilities. It is still in a process of prolonged transition from the pay-as-you-go scheme of old to the objective of being fully funded by 2019.
Meanwhile ACC's levy income rose $230 million to $4.8 billion, largely reflecting higher levies in the work and earners' accounts.
The bottom line was also assisted by lower rehabilitation and compo costs, which fell $100 million and $180 million respectively.
This is the area which ACC itself can take credit for, as distinct from external factors such as the state of world financial markets and political decisions like levy increases, changes to the coverage of the scheme and pushing back the target date for full funding.
Rehabilitation rates - how long on average people stay on ACC - have improved a lot over the past couple of years, to the point where the number of people on ACC for more than a year is the lowest since comparable records began in 1994.
Rehabilitation rates had deteriorated markedly between 2004 and 2009, from the relatively high rates achieved in 2002 and 2003 after the renationalisation of the work account, when the Government and the corporation had something to prove.
It has rediscovered the financial, as well as human, benefits of front-loading support to the injured.
But it is not only that. ACC has had to be "firmer" on getting people back to work, ACC Minister Nick Smith said. "It's better to get GPs to focus on what people can do rather than what they can't."
Between 2005 and 2008 the number of compensation claims grew significantly faster than the working population; since then it has fallen in both absolute and per capita terms.
This illustrates what last year's "stocktake" review of the scheme, led by David Caygill, a former ACC chairman, saw as its inherently cyclical dynamic, alternating between periods of laxity and stringency.
"The history of the ACC scheme to date is one of recurring crises resulting from rapid and unaffordable expansion of the claims liabilities, followed by periods of greater focus on claims management and rehabilitation," the stocktake report said.
So long as the monopoly structure of the scheme was maintained there was no guarantee the operational reforms the stocktake recommended would prove durable, it said.
Smith says that without the constant pressure of competition ACC will inevitably revert to a cost-plus culture.
Conveniently for him, it was the work account, the one which covers workplace accidents and is earmarked for opening up to competition, that was the best-performing part of ACC in the latest year. It provided nearly half of the $1 billion increase in net surplus, even though it represents about a fifth of ACC's total business.
It leaves the work account close to being fully funded, with enough reserves to cover the net present value of its liabilities.
Less conveniently, however, the $494 million lift in the work account's surplus is almost entirely explained by a $290 million increase in levy income (reflecting the first full year of the levy increases announced in 2009 and implemented in 2010) and a $200 million improvement in its investment income. Advocates of change have argued that the scheme as it stands does not provide enough incentive for employers to lift their game in terms of injury prevention or support the rehabilitation of injured workers. The industry classification groups that firms are bundled into are too broad, allowing cross-subsidy from low-risk to high-risk employers.
Opponents point out that a fifth of the workforce is in firms in accredited employer schemes that self-insure wholly or in part.
And this year ACC has reinstated experience rating, precisely intended to provide more fine-grained pricing and improve incentives.
How about seeing how well that works before making more radical changes?
An obvious risk with opening workplace accident insurance to competition is that private sector insurers will cherrypick, targeting low-risk employers and leaving ACC as the default provider for high-risk ones.
"This could create an ongoing deteriorating pattern which could make the ACC work account unsustainable," said the Department of Labour, which oversees ACC, in its response to the stocktake report.
"If this occurred it may require an injection of funds from the Government to cover outstanding liabilities."
Private insurers face costs ACC avoids, in particular the need to provide shareholders with a return on their capital and to pay tax, which ACC as a Crown entity does not. The Labour Department reckons they would have to fund savings in claims cost in the 20 to 26 per cent range to offset their higher expenses and keep levies similar to today's levels.
Smith believes these two risks - that private insures will cherrypick or that they will struggle to compete against the advantages ACC enjoys - roughly balance out.
But of course it could avoid both of them by just leaving the scheme alone.
