The United Church of Canada is a creature of statute and, as such, must comply with the statute.....

the Basis of Union is the authority under which the United Church of Canada functions

The Basis of Union set forth in Schedule A to this Act is hereby ratified and confirmed as such, and in so far as the terms and provisions thereof relating to polity and administration are not inconsistent with the provisions of this Act they shall have the same force and effect as if expressly set out herein. [The United Church of Canada Act, section 26];

The United Church of Canada Act authorizes only lawful acts or things:

To do all such lawful acts or things as may be requisite to carry out the terms, provisions and objects of the Basis of Union and of this Act. [section 18(j)];

The United Church of Canada Act authorizes:

To make such by-laws, rules or regulations as it may deem expedient for the exercise of any powers conferred by this Act. [section 18(h)];
Notwithstanding anything in this Act contained, it is hereby declared:-
That nothing in this Act contained shall be deemed to limit the independent and exclusive right and power of The United Church to legislate in all matters concerning its doctrine, worship, discipline and government, including therein the right and power from time to time to frame, adopt, alter, change, add to or modify its laws, subordinate standards and formulas and to determine and declare the same or any of them, but subject to the conditions and safeguards in that behalf contained in the Basis of Union. [section 28(b)]; and

the referenced conditions and safeguards are:

The General Council shall have full power:
(1) to legislate on matters respecting the doctrine, worship, membership, and government of the Church, subject to the following conditions:
that before any rule or law relative to these matters can become a permanent law, it must receive the approval of a majority of the Presbyteries, and, if advisable, Pastoral Charges also...
that no terms of admission to full membership shall be prescribed other than those laid down in the New Testament;
that the freedom of worship at present enjoyed in the negotiating Churches shall not be interfered with in the United Church;
(2) to legislate on all matters respecting property, subject to the limitations elsewhere provided in the Basis of Union, and subject also to the approval of the Conference in which the property is situated; [basis 8.6, 8.6.2]

the United Chuch of Canada is conciliar, not hierarchical, in government such that each court has specific rights and responsibilities which may not be assumed by any other court:

A. Charges Existing Previous to the Union
In the management of their local affairs, the various churches, charges, circuits, or congregations of the negotiating Churches shall be entitled to continue the organization and practices (including those practices relating to membership, church ordinances, Sunday schools, and young people’s societies) enjoyed by them at the time of the Union, subject in general affairs to the legislation, principles, and discipline of the United Church... [basis 5.1]
Churches, charges, circuits, or congregations received subsequent to the Union, into the United Church, with the approval of Presbyteries, shall be entitled, if they so desire, to the privileges of sections 5.1... [basis 5.5; see also section 8 of The United Church of Canada Act where congregations received subsequent to the union were not differentiated from negotiating congregations and section 26 of The United Church of Canada Act "in so far as the terms and provisions thereof relating to polity and administration are not inconsistent with the provisions of this Act" with reference to the word "entitled"]

It shall be the duty of the Session to have the oversight of:
(1) the admission of persons into full membership, their removal, and the granting of certificates of transfer;
(2) the conduct of members, with power to exercise discipline;
(3) the administration of the sacraments;
(4) the religious training of the young, and the organization of meetings for Christian fellowship, instruction, and work;
(5) the order of public worship, including the service of praise and the use of the church edifice; and
(6) the care of the poor, and the visiting of the sick. [basis 5.10.1]