The $582 charge was calculated by taking the $190 fixed annual charge that all Watercare customers pay and adding the average volumetric usage of a two- to three-person household.
The decision to hold a trial was made after pressure from the Whangaparaoa Residents & Ratepayers Association.
President Winsbury White, an advocate and volunteer for tank meters, is yet to get a bill to see how actual use stacks up with the company's estimate.
He expected savings to recoup in time the $450 to have the meter fitted.
But he realised the idea would not suit everyone. Water needs had changed on the Whangaparaoa Peninsula during his 33 years living there, with many baches being transformed into modern homes.
He did not think that families would be keen to adopt meters unless they were conservative with water use.
"I suspect a family of three or four would use more water than the $582 charge anyway.
"But we don't waste it. When we turn on a tap we are mindful about how much is going down the drain."
Mr White disputed the fairness of Watercare's calculation that an average two- or three-person household put 113,150 litres a year down the drain.
He said it charged some tank owners for thousands of litres of water that they did not put down the drain. It penalised those who conserved water or had holiday homes or small houses with a lesser roof collection area and tank.
A meter on the tank was not asking any more than was already done for Watercare customers, with their metered town water supply.
Since July, they have paid $1.343 per 1000 litres for drinking water.
Most of their wastewater charge is based on the assumption that 78.5 per cent of water used goes down the drain to the sewer.
Watercare said it did not intend to meter rain collection tanks if they were for only watering the garden.
The figures:
8256 homes rely on rain tanks
$582 annual fixed charge to use sewer
14 homes trial tank meters to check sewer use
$450 for meter & fitting
