It is seeking consent until 2044 and says it needs "operational certainty".
The application, prepared by consultants Boffa Miskell, says there has been a thorough investigation of alternative options to further manage stormwater and treat contaminants.
"This has considered all practicable options to manage stormwater within the constraints of the flat site and security requirements of the prison. Alternatives considered included a complete diversion of the discharge to the outfall of the lake system to a point downstream of Lake Wiritoa.
"This option was discounted as being of significant cost with low engineering feasibility. Disposal to the current outlet at the channel connecting Lakes Pauri and Wiritoa, with the installation of additional treatment before discharge, was concluded in the options assessment as the only practically available method to manage stormwater from the prison."
Dr Vaughan Keesing, of Boffa Miskell, has prepared an ecological assessment on the effects of the stormwater discharge on the ecology of the lakes, the application says.
"This report concludes that the receiving wetland and lake habitats flora and fauna are already modified and reduced in condition.
"Dr Keesing concludes that the mitigation measures already undertaken and 'UpFlo' treatment device proposed will greatly improve the water quality of the discharge, therefore improving the life supporting capacity of the receiving environment.
"In making this conclusion, Dr Keesing notes that the prison discharge is however a relatively small contributor to the water quality of the receiving environment and that other land uses surrounding the lakes have the greatest impact on their water quality."
Both lakes are now subject to algal blooms.
The lakes are prized by Tūpoho and Ngā Wairiki/Ngāti Apa, and an independent cultural assessment has been done.
"Consultation with Ngāti Tupoho and Ngā Wairiki Ngāti Apa on the discharge undertaken in 2018 identified the opportunity for Corrections to work with tangata whenua to improve the environment around the discharge channel by way of a new project to remove exotic species and weeds replanting with native wetland and lake edge species, utilising the large nursery at the prison," the application says.
"Corrections have confirmed a willingness to work with both Ngāti Tupoho and Ngā
Wairiki Ngāti Apa on this initiative. This consultation confirmed support for the discharge proposal by these mana whenua groups, in particular the work undertaken and proposed to further improve the quality of the discharge which is part of this application."
The application and associated reports are available online at horizons.govt.nz and at Horizons' offices in Whanganui, Marton and Palmerston North. Submissions closed on October 26.
The resource consent hearing starts at 9am on Tuesday, November 17, at 151 on London Motel and Conference Centre in Whanganui.
