From Auckland 
via Whangarei 

Head north, staying on State Highway 1.
You will pass through Whangarei and Kawakawa, and an interesting tee-intersection at Ohaeawai that catches out the odd visitor!  (If you weren't watching the signs at this tee-intersection, you would assume you should continue straight, when in fact State Highway 1 goes off to the right.)
Turn off State Highway 1 at the Mangamuka Bridge, which is 36k past Ohaeawai (or 63k further on from Kawakawa.)
This turn-off at the Mangamuka Bridge is signposted as "Rawene (via ferry)" and "Broadwood".  From the turn-off it is about 25 minutes to Kohukohu - just follow the signs and enjoy the Hokianga scenery!

To find your hosts, Lex and Wichard, just follow the main road in Kohukohu and turn right into Maning Street, which is the second road on the right past the school grounds. The entrance to our house is right opposite the police station.
 

Head north on State Highway 1 and turn left onto State Highway 12 at the Brenderwyns just north of Kaiwaka. Follow this route through Dargaville, the Waipoua Forest (with its giant Kauri) and Opononi at the entrance of the Hokianga Harbour. Take the Rawene turn-off 17 km past Opononi.  The north-bound Rawene - Kohukohu Car Ferry leaves every hour on the half hour from 7.30 am until 7.30 pm.
See more notes below on Car Ferry info.
Arriving on the north side of the harbour it is 3km to Kohukohu.

To find your hosts, Lex and Wichard, follow the main road into the village for about 150 metres and turn left into Maning Street. The entrance to our house is right opposite the police station.
 

From Kaitaia follow State Highway 1 south for 37km and turn right at Mangamuka Bridge - further directions as above from Auckland via Whangarei.
Alternatively from Kaitaia, follow the Twin Coast Discovery Highway towards Ahipara and then through Broadwood to Kohukohu.
Again, enter Kohukohu as the above Auckland via Whangarei directions.
 

Air New Zealand fly Auckland to Kerikeri 5 times a day Monday to Friday, and 3 times a day on Saturday and Sunday. Kerikeri Airport is 50 minutes drive away from Kohukohu - pick up is available but charges apply.
Kerikeri Airport has a good choice of Rental Cars.

Notes on the Car Ferry:

The north-bound
Rawene to Kohukohu
Car Ferry leaves
every hour on the half hour
from 7.30 am until 7.30 pm

By the way, if you are planning to use the ferry more than two times during your stay it is cheaper to buy a $30 concession ticket for 5 trips.