Bills are now monthly instead of the three- or six-monthly invoices many Aucklanders have been used to.
"My tenants are frugal with water. Then I noticed the consumption figure was an estimate," he said yesterday.
His bill showed readings for consumption at the house of 69 kilolitres (69,000 litres) over 246 days, including summer months - 280 litres a day.
It said estimated consumption over 29 days in August would be 29 kilolitres, or 1000 litres a day.
"Even their own graph of usage history should have given them a clue as to their cock-up," said Mr Osborne, a former maths teacher.
"The bar at the right of the graph is nearly four times the height of the two bars on the left.
"The effect of such a glaring overestimate is to give Watercare an interest-free loan for a month or more - in this case two months."
A Watercare spokesman said the billing team's estimate relied on a previous month's reading by a meter reader, who could not see all the figures properly because of condensation in the meter. It would be checked by a serviceman and Mr Osborne would get a revised bill, showing a more appropriate estimate.
Said Mr Osborne: "They apologised, saying it was human error, but I was bothered by the fuss and the 40 minutes it took me on the phone to sort it out."
Last month, householders received a "transition" bill as Watercare slowly brings customers of the previous local authority water retailers under one system.
From July 1, Watercare increased the price of drinking water by 4c per 1000 litres, or 3.3 per cent, to $1.343.
Wastewater charges have changed too and customers pay a $190 fixed service charge per water meter plus a volume charge of $2.281 per 1000 litres.
