Graeme Samuel, the chairman of the ACCC, was more inclined to blame the drought and “other factors” pointing out also that in fresh food the market share of Woolworths and Coles was probably less than 50%.

I heard Choice supporting Zumbo and the Retail Association supporting Samuels. Stephen King doubts the duopoly argument and points out that prices have risen faster in Britain in the last five years.

I had a look at CommSec research to find that Wesfarmers, owners of Coles, had a net profit margin of 3.2% while Woolworth’s was similar but slightly higher at 3.7%. I know an investment adviser who won’t buy a company unless these values are up around 12-14%.

Wesfarmers, of course, owns other businesses, notably a couple of coal mines. But it is now largely a retailer, with Coles the largest business. One would think that sans the coal mines margins would be lower. Woolworths also owns other businesses albeit all retailers as far as I know.

I think if you asked farmers and other producers they wouldn’t class either of the retail giants as generous in what they pay.

Both are increasingly being kept honest by Aldi.

Roy Morgan has recently had a look customer satisfaction in the grocery sector. Coles and Woolworths are shaded by Aldi, but satisfaction rates of about 85% seem to me quite high. The banks would dream about levels like those.

I note that Prof Zumbo has offered no proof for his assertions about the duopoly as cause of the price hikes. He may have proof, but all we’ve got is assertion. Whatever the reasons, and I have no idea, there seems to me no evidence of net price gouging by the big players.

Something has been made of the fact that we now import more of our horticultural produce, now being net importers. But I seem to recall that the Australian dollar was very low around 2000, down around the 50c mark. This should make imports cheaper now.

Perhaps nthere was some cherry picking, perhaps inadvertent, in choosing the interval Zumbo chose. This article states that world food prices rose by 75% from 2005 to 2008.

In other supermarket news, unit pricing has been rolled out in the local Coles and Safeway. That is, just about every product line in the shop now has a second label telling you the price per 100 grams, or 100 millilitres. These tags are supposed to make it easier to compare the price between different-size bottles from different (or the same) brands, and thus let people choose the cheapest if they so desire. It’s fairly standard Economics 101 – make the market more transparent and it’s supposed to work more efficiently.

But, of course, a “4 for 3 special” means that the unit price on the sticker on the shelf no longer applies and means the shopper is back to a bit of mental arithmetic. Am I being paranoid, or is this not entirely coincidental? And if so, what’s in it for the supermarkets?