Devonport is mostly flat from the ocean to the centre of the city. There are some hilly sections but if you don't live in those areas then there is not a problem.
There are parts of Devonport that are hilly, but the CBD along the Mersey, and the areas along the Bass Strait beaches are very level. We are in our 60-70s and find the area perfect for walking. We walk to the CBD for shopping, cafes and entertainment, and walk the coastal trail for exercise. We are KIWIs who had been living in the Pacific Northwest of USA. Planned to retire to the South Island of NZ but fell in love with Devonport and the Tassie Northwest. Good luck!