Every January, Oxfam releases a report on global wealth inequality. This year's Oxfam report contrasted the drop in wealth held by the less wealthy half of New Zealand with the rise in wealth enjoyed by the two Kiwis who made it on to Forbes' 2018 list of billionaires.

The report, and the Stuff story on it, suggested that New Zealand's rich have got richer while the poor have got poorer.

But Oxfam's reports rely on the Credit Suisse annual reports on global wealth, released every October. The actual Credit Suisse figures vary considerably from how Oxfam has presented them.

New Zealand's less wealthy half is indeed less wealthy than in 2017's figures. Median wealth, the amount of wealth held by the middle person in the wealth distribution, measured in US dollar terms in Credit Suisse's report, dropped from $101,718 to $98,613. So the wealth of the richest person in the bottom half dropped.

But the Credit Suisse report shows that New Zealand's wealthier have also got poorer. Average wealth, which is more sensitive to changes at the top, dropped from $300,988 to $289,798.

And despite New Zealand's small population, we made the list of countries with the biggest drop in the number of people in the global top 1 per cent. Twenty thousand fewer New Zealanders counted themselves among that global elite in 2018 than in 2017.

So, contrary to the Oxfam report, both rich and poor became less wealthy last year.

If you had not noticed that we had all become a bit less wealthy, you can be forgiven. Credit Suisse tallies global wealth by putting everything into US dollar terms. The New Zealand dollar depreciated against the US dollar last year, reducing the value of all assets denominated in New Zealand dollars.

This year's drop followed last year's increase in reported wealth, which was also due in part to exchange rate movements.

Oxfam's report reached a vastly different conclusion to the underlying Credit Suisse figures because Oxfam ignored half the data in the Credit Suisse report. The contrast between the wealth increase for two Forbes billionaires and the wealth decline in the Credit Suisse figures was painted as an increase in overall inequality.

But the wealth holdings of two billionaires, in a country of about five million people, really do not tell us much about what is going on overall.

Credit Suisse's report uses the Gini measure of inequality in wealth holdings. The Gini measure compares everyone with everyone else rather than relying heavily on two billionaires.

Credit Suisse's figures show that wealth inequality dropped from a Gini figure of 72.3 in 2017 to 70.8 in 2018. New Zealand currently sits between France and Canada – about where you might reasonably expect.

Discussing the report on Newstalk ZB, Oxfam's Rachael Le Mesurier said "this gap is getting bigger".

According to Newstalk, "she said the gap had been continuing to grow since the inception of their reports about five years ago". But the Credit Suisse 2013 wealth report, which would have formed the basis for Oxfam's 2014 report, had New Zealand's wealth Gini coefficient at 71.8 – again, less equal than 2018's figure.

People can reasonably disagree about how much redistribution is appropriate. The forthcoming Tax Working Group report will begin a lengthy debate about how and whether the overall tax system should change. That debate will not be helped by misleading statistics. We should expect better from Oxfam.