The online information has been removed, more checks and assessments are to come, and the adopted rates totals will be up later in July.
"We apologise for this error and any concern this has placed on our ratepayers," Macdonald said.
Peake can live with paying $3.74, and is pleased he picked up the error.
The Matarawa Stream flows through Whanganui East and its management scheme aims to stop it flooding residents. Upstream there are diversions and dams, but within the suburb it had become clogged with vegetation.
Last summer Horizons spent $320,000 clearing part of it. The rest is to be cleared this summer.
The clearing will be funded by a loan, to be paid back over 10 years, with the cost shared by all urban Whanganui landowners. After 10 years it will halve, because only maintenance will be needed.
For Whanganui ratepayers rates have increased an average 6.95 per cent this year, Macdonald said. The only new rate is one for overseeing the health of drinking water.
Pre-existing rates have gone up by varying amounts. Targeted rates, which are mainly for flood protection, have gone up by an average 20 per cent.