The agency will launch a multi-channel, four-week campaign this autumn aimed at persuading visitors to use the bus or train when visiting the park.
It will seek to show how easy it is to reach walking and cycling routes via public transport, with reference to discounts on bus and train access.
Cheryl Campsie, associate director at Forster Communications, said: "We need to inspire our audience of couples who visit the park to walk and cycle to give the bus and train travel a go, through trials, incentives and tackling negative perceptions.
"We will highlight the surprising elements of public transport – that is it easy and relaxing - and the benefits such as the stunning views of the National Park from the window and enjoying a guilt-free drink at the end of a walk."
Nick Stewart, SDNPA campaigns and identity project manager, said: "We have ambitious targets to change the travel behaviour of different groups of visitors to the park. Forster Communications has already proved it has real insight into audience barriers and motivators that can drive change and bring about the shift we need in travel behaviour."
The project follows previous collaboration between Forster and the SDNPA.
It is part of the successful joint bid to the Department for Transport Local Sustainable Transport Fund for £3.81m, led by Hampshire County Council in partnership with the SDNPA, New Forest National Park Authority and neighbouring highways authorities across the two National Parks.