Cr Burdett said it was important for the council to control the critical asset.
All the land where the existing wastewater plant was located was owned by the council.
Cr Meredith Akuhata-Brown asked how the recommendation would require rates to rise to cover interest and principal payments.
Council chief financial officer Pauline Foreman said the effect on rates would be less than if the properties were leased.
Cr Pat Seymour drew attention to the recommendation from Ms Foreman that the costs of interest and principal would be lower than any payments owed to GHL.
Councillors agreed to approve a forecast debt for 2020-2021 of $95.7m to enable purchase of the Banks Street properties although that was outside the debt threshold of $85m for years 1, 2 and 3 of the 2018-2028 long-term plan.
They agreed not to amend the debt threshold over that period as there is a higher debt provision of $105m in year 4 to accommodate costs from the WWTP disinfection stage.