Paihia and Russell are the main towns in the Bay of Islands area. They are connected by road (a long and scenic but dangerous drive), a car ferry between Opua (6 km south of Paihia) and Okiato (12 km south of Russell).
Most visitors will use the passenger ferry across the bay between the two cities. There are three companies operating these ferries and they run more or less every twenty minutes till 10 pm. It takes about ten minutes to cross from Paihia to Russell.
A ferry crossing cost NZD 5,- one way. I would recommend to buy one way tickets, so you are free to go back with each company.
With holidaymakers staying in Pahia or in Russell and wanting to make daytrips to either destination several ferry services started up. Some are high speed and others are slower but take about 15 minutes maximum. The ferries run very regularly from both locations. These ferries are for foot traffic only - no cars. If you want to drive between the two places then you will have to go back to Opua and take the car ferry across to Russell that way.
Most people travel to the Bay of Islands by car. To get to Russell the best way is to drive to Opua and take the car ferry across. It costs about $18 return for a car and driver. The ferries are run by Fullers and run more frequently during peak times. You might have to wait but usually no longer than about 30 minutes depending on traffic.
Opua is located just after KawaKawa.