The occupation of northern Indochina at least had the pretext that it would stop supplies going to China. Landing in French Polynesia is a clear indication that Japan intends to do something about the European colonies and threaten Australia and New Zealand. It would cause a reaction in the British Empire and the United States. And it would greatly impact the Free French and Vichy France. Remember that the move into northern Indochina technically was agreed to by Vichy France and the French authorities remained in place even though Japanese troops were everywhere.
Instead of being considered a backwater, the British would have to take defense plans in Asia seriously. Secondly, if the Japanese land in New Caledonia, it may prompt the British to occupy French Polynesia to beat the Japanese to prevent a fait accompli. Fighting some cut off French forces in chaos is one thing, but having to kill New Zealanders or Australians to take Noumea means war with the British Empire in 1940. If the Japanese have arranged things to occupy everything at the same time to prevent that, it'll just spook the US and Britain even more. Hard to see how any surprise attack could happen after that. Don't forget that right in the middle is the New Hebrides - an Anglo-French condominium. That's going to get fortified very quickly.
If this occurs immediately after the armistice, it puts Vichy France in a poor situation. If it can't protect French colonial possessions, it risks alienating the rest of the colonies that they may declare for Free France. It might even butterfly the attack on Mers-el-Kebir as the French Fleet defects to the Allies so that it can be deployed to protect the French colonies. If French North Africa declares for Free France, it will help the Allies far more than losing the French Pacific possessions hurts them.
This is going to erode isolationist sentiment in the US even faster. US planning with the British just won't be about helping them in Europe, but preparing for things in the Pacific. I imagine the US preparations in their island possessions and the Philippines will be much more thorough and done under a greater sense of urgency.
The new Japanese possessions are very, very far away from their other logistical base of support. In case of war, they will likely fall very quickly unless you expect the Japanese to waste naval assets to protect them. To my eyes, this is done just way too early to give them any benefit compared to what they lose in regards to shocking the Western democracies.