The company is progressing plans to build the reservoir on land it owns between Staunton Country Park and Havant Thicket.
The project, a partnership with Southern Water, would secure much-needed water supplies for the South East, create a new, public leisure space for nearby communities and a wildlife haven with wetlands for birds
At the drop-in session, residents will be able to talk to members of the project team who are developing the plans for the reservoir, ask questions and share ideas and feedback. Visitors will also be able to see how the reservoir may look in an interactive model.
There will also be information about the soil and water investigations which are taking place on the site in November and December.
The drop-in session is being held on:Saturday November 23, 12.30pm to 4.30pm, at Leigh Park Community Centre, Dunsbury Way. Havant, PO9 5BG.
Bob Taylor, Chief Executive of Portsmouth Water, said: 'We want to give local residents and businesses the opportunity to find out more about the reservoir and hear back from them before we take the next steps to develop our plans.
‘The reservoir would be a huge asset to the local area and we hope residents will get involved and play a part in helping us create an amenity that is valued by the community and enhances the environment.
‘What we hear will help inform the design and planning application, which we intend to submit later next year, and ensure we can plan to carry out the construction in a way that has minimal impact on the people of Havant.'
As well as sharing information about the reservoir and investigations, Portsmouth Water employees will be on hand to provide water efficiency advice and share information about water meters and support for customers with their water services and bills.
The new reservoir would be the first one to be built in the South East since the 1970s and is being developed in partnership with Southern Water.
Contractors have set up on the site and investigations on the soil and groundwater will be carried out this month and next month. This will involve creating temporary boreholes and shallow excavations on the grassland. The work will avoid key areas of sensitive wildlife habitat and the wooded areas.
During the work, the footpaths and public bridleway will remain open and working hours will be restricted to between 8am to 6pm, Monday to Friday, and 8am to 1pm on Saturdays.
Portsmouth Water is advising people to take extra care around the site, supervise children and dogs and to keep a safe distance from the machinery.
Find out more about the reservoir online at portsmouthwater.co.uk/havant-thicket-reservoir or follow the reservoir Facebook page @HavantReservoir.