It would basically be illegal, regardless of what a contract says: contracts are subordinate to the law. So the question comes down to whether the law unequivocally requires OT for shifts longer that 8 hours. These sorts of governmental fact sheets are not absolute statements of the law, they are guidelines about what is most commonly applicable, and to determine if there are any exceptions, one must consult the actual law. The fact sheet says "After working eight hours in a day an employee must be paid time-and-a-half for the next four hours worked", which is somewhat different from "in a shift". The (summarized restatement of the) law says that an "averaging agreement" is legal (if in writing), so if that allows working more than 8 hours in a shift, that could be consistent with (does not override) the law. To really know, you have to read the actual law.
The section on maximum hours and overtime says
35 (1) An employer must pay an employee overtime wages in accordance
with section 40 if the employer requires, or directly or indirectly
allows, the employee to work more than 8 hours a day or 40 hours a
(2) Subsection (1) does not apply for the purposes of an employee who
is working under an averaging agreement under section 37.
So 3 shifts of 12 1/3 hours would be legal, and perhaps typical in a hospital. The "average hours" part of the law says
an employer and employee may agree to average the employee's hours of
work over a period of 1, 2, 3 or 4 weeks for the purpose of
determining the employee's entitlement, if any, to overtime wages
under subsections (4) and (6) of this section and wages payable under
subsection (8) or (9) (b).
as long as there is a clear advance agreement (as in a contract). This still limits hours worked without overtime to 40 hours/week (on average), and then there is still an upper limit of 12 hours:
An employer under this section who requires, or directly or indirectly
allows, an employee to work more than 12 hours a day, at any time
during the period specified in the agreement, must pay the employee
double the employee's regular wage for the time over 12 hours.
Since the law is stated in terms of "day", we better see what a "day" is:
(a) a 24 hour period ending at midnight, or
(b) in relation to an employee's shift that continues over midnight,
the 24 hour period beginning at the start of the employee's shift;
so you can't force an employee to work 12 hours without overtime (without an averaging agreement), simply by splitting a 12 hour shift across midnight.
It is thus possible for a contract to do things that superficially appear to be contrary to the "most common situation" factsheet representation, but the law itself is not overridden by such a contract.