The ferry from Auckland's Quay Street terminal to Devonport runs frequently throughout the day (roughly every 30 minutes, more often in peak hours) and takes 12 minutes. This is by far the best way to arrive — cheap, fast, and atmospheric, with good views of the harbour and the city skyline. Fullers operates the service; buy tickets at the terminal or on the Fullers app.
By car, Devonport is reached via State Highway 1 and then local roads through the North Shore — allow 30–40 minutes from the city centre depending on traffic, plus parking time. The ferry is generally much quicker.