You have two options: either way is 2 hours driving.
1) Queen Charlotte Drive - scenic but narrow, windy road around the coast.
From the Picton Ferry terminal take State Highway 1 (to Blenheim/Christchurch) for just a few metres, then turn the first right into Queen Charlotte Drive to Havelock/Nelson. If you like mussels The Mussel Pot in Havelock is a good place to stop.
About 20km after Havelock you will come to Pelorus Bridge. If you have the time a 20 minute walk through the Totara Trail is a great way to stretch your legs. There is also a café here.
As you come into Nelson, the signs at the roundabout will point you left to the City Centre. Turn the next left into Wainui Street, then the next right into Collingwood Street, and the first left into Cambria Street. Cambria House is the third house on the left- No 7.
2) Via Rapaura Rd- more kilometres but a good straight road.
From the Picton Ferry terminal take State Highway 1 for 22km until you reach Spring Creek and the by-pass to Nelson. Turn right at this sign onto Rapaura Road. At the end of Rapaura Road turn right onto State Highway 6 and follow the signs to Nelson. There are vineyard and shopping opportunities on Rapaura Rd.
Follow the signs to the City Centre via Tahunanui at each roundabout. As you drive past the gas tanks at the Port, stay in the left lane, ready to take the exit onto Queen Elizabeth II Drive as if you were going to Blenheim or Picton. Over the bridge, and at the roundabout, take the 3rd exit onto Trafalgar St. Turn left at Wainui St. right at Collingwood St. and left into Cambria St. No. 7 is on the left. Welcome to Cambria House.