A very interesting topic, which can be cleared up by visiting the home page of the United Nations, and a search through the United Nations Charter will reveal some interesting information.
The United Nations consists of three main bodies:
The General Assembly, the Security Council, and the Secretariat
The General Assembly - of which all nations of the world may wish to join as ordinary members, to benefit from the resources of the various programs developed by the UN.
The Security Council - form with Five Permanent Members - (China, France, Russia, UK, and USA) - and Ten Non-Permanent Members from the General Assembly who will sit for a two year term.
The five permanent members carry the right to veto any vote.
All members at the General Assembly are independent nations, and each country's sovereignty are respected by all members, and are willing to submit themselves to the rulings of the United Nations.
The General Assembly will select from their ranks, ten members who will be vote carrying members to represent the General Assembly at the table of the Security Council, and will sit with the other five Permanent Members to oversee Security Issues affecting the entire World.
The role seems to be quite well defined between the General Assembly and the Security Council.
The General Council will concern itself with the elevation of the living standards of the world population so as to maintain World Peace, by promoting progress in development through economic, social, cultural, health, education activities.
The Security Council has been given the role to oversee and deliberate any necessary response to maintain security in the World, and operates without recess through the year.
The Secretariat, in which the UN Secretary General has his office, will see to the smooth functioning of the United Nations, advising the General Assembly of the proceeding at the Security Council, and to update the member states when the General Assembly is in recess.
There is no question of the General Assembly trying to overturn any decisions made at the Security Council, as ten of the fifteen members at the Security Council are representing the members of the General Assembly.
To change this Charter that affect the current United Nations, it will require the General Assembly to pass a motion with two-thirds majority, and the Secuity Council to approve the same with a vote of nine out of fifteen.
From the following website, see the following concerning:
http://www.un.org/aboutun/charter/General Assembly - its role and authority
Chapters: IV, IX, X
Security Council - its role and authority
Chapters: V, VI, VII, VIII, XII
The Secretariat - its role and authority
Chapters: XV
Particular relevance to Singapore in the Water Dispute with Malaysia is Chapter XVI, Articles 102 and 103.