Guadalcanal and Japan's threat to Australasian shipping

Did Japan seriously try to block shipping lanes from the United States to Australia and New Zealand?

Yes; the Guadalcanal campaign offered a way to threaten ports and normal sea lanes with land-based aircraft, forcing Allied shipping onto longer routes.

Japan did not need to patrol every mile between the United States, New Zealand, and Australia. Establishing a strong air base on Guadalcanal would extend maritime patrol and strike aircraft across the northern and central approaches. Threatening Australian ports and the normal routes could force shipping far to the south, imposing distance and delay even without a perfect blockade. This operational logic is one reason Guadalcanal became so important.