But Horizons principal adviser Tom Bowen said that shouldn't be relied on.
New Zealand's climate policy is shifting toward reducing emissions rather than emitting and then offsetting them.
The inventory cost $87,000 and was not required, but Bowen said the public increasingly expects emissions to be counted, and inventories could become a requirement.
Getting carbon credits from the joint-venture forestry is not a certainty either, councillor John Turkington said.
"Some of the forestry ventures were planted under the Afforestation Grants Scheme and government takes the first 10 years of carbon from those."
An accountant will check how much carbon the council can actually claim, and report back in November.
With council vehicle use accounting for more than half the emissions, changing to hybrid or electric vehicles is an obvious way to reduce emissions, Turkington said - but not if the electric vehicles are powered by electricity made by burning coal.
"We have clearly got a generation problem in this country. Government is promoting electric vehicles, but it's a complete waste of time if we have got to burn coal to create the electricity."
Councillors have signalled they want to develop a responsible investment policy, Bowen said.
Councillor Allan Benbow said the council planted a lot of trees, and should find a way to get carbon credit for riparian plantings.
Whanganui councillor Nicola Patrick was frustrated it has taken so long to get an inventory, and looked forward to workshops about how to reduce emissions.
Emissions will continue to be counted and audited, and the council has set a goal of reducing them 30 per cent by 2030.