That slogan, from the river to the sea, is an implicit call for the destruction of Israel, and inevitable genocide (again) of the Jewish people who live there. It is a slogan used by Hamas whose charter calls for the destruction of Israel.
The slogan is a rejection of a two state solution and a call for Palestine to be established on all the land between the Jordan River and the Mediterranean Sea. This is a rejection of both the 1948 UN mandate and the 1967 boundaries recognised by the UN Security Council.
I need not remind people the Jewish people have already faced one genocide. To have a New Zealand MP tweeting a slogan that calls for the destruction of their homeland is terrible.