From the Tourist Office in Nelson, obtain directions for the cycle path toward Motueka. This is a pleasant, flat cycle alongside the Tasman Bay and will cut out most of the urban streets as you leave the town. The road is generally flat and pleasant enough but at the end of the cycle path, you might like to break up the day by heading slightly inland and taking the country route through Upper Moutere. This is slightly further than the main road, but is pretty countryside along less busy roads. Another option is to turn off at Mapua and follow the quieter (since the completion of the Ruby Bay Bypass in 2010) coast road through Ruby Bay, rejoining the coastal highway just after Tasman.
Motueka is a much smaller version of Nelson in many ways. There are plenty of arts and crafts, a supermarket and two good camping grounds and hostels. It's a fine place, but I must admit to wanting more mileage out of my day and usually head a further thirteen kilometres to Kaiteriteri.
The campground here is right on the beach, and the beach is beautiful. It has safe swimming and fine sand. The short local coastal road here will take you up to Marahau, a lovely, tranquil ride with beach and bush views. Marahau has a good campground, store and a café and there is a side road (a fairly steep ride) to take you up onto the main road to Tākaka. If you are feeling lazy, you can get a launch from Kaiteriteri all the way to Totaranui on the far side of the Abel Tasman National Park and avoid the Tākaka Hill altogether. Launches are also available from Marahau.
There is a campground at Totaranui and the road, though shingle, winding and undulating, will take you the last 32 kilometres through to Tākaka. The Abel Tasman National park is one of the most beautiful reserves in the country. You can store the bike for a few days and walk the blue bays around this magnificent coastline. There are campsites and a fine coffee bar/restaurant at the far end of your hike. However, if you want to cycle on, you can head west over the Tākaka Hill.