When one or more lanes end or where two lanes join into one or where a feeder lane (e.g. an on-ramp or slip road) joins another road, vehicles need to merge. The rule for merging where a lane ends is that you must signal your intention for at least three seconds before performing the manoeuvre. This is covered in clause 3.10(3) and 3.10(4) of the Land Tranport (Road User) Rule 2004.
In some situations, two lanes merge but neither lane actually ends. It’s not legally required for you to indicate, but it is best practice because other drivers might not have noticed that the lane is merging (i.e. the road is narrowing) and therefore may not be expecting you to move left or right.
Merge ‘like a zip’, i.e. vehicles merging should slot alternately between vehicles already in the target lane (assuming traffic is heavy enough to warrant it. You should be using the feeder lane to get up to the same speed as traffic in your target lane. Once you have merged, adjust your speed and following distance.
Signs that indicate a lane is ending or joining another and that you will need to merge are: