Devonport is easily reachable by ferry from Auckland City.  You can find the timetable for the Devonport Ferry here. During the day the ferry leaves every half hour from behind the iconic downtown Ferry Building: Pier 1, Ferry Terminal, 99 Quay Street.  The ferry trip takes approximately 15 minutes.

You can also get to Devonport on Stagecoach buses, you can use routes: 
779, 780, 813, 815, 816.

The MAXX Journey Planner is an effective way to find the most appropriate bus route for you.