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Within the no-circling area, as the required obstacle clearance at the MDA is not provided, conditions need to be such that the pilot can see and avoid obstacles.  For that reason, circling should not be conducted within the no-circling area unless by day and in VMC.  
In some cases the missed approach point is located within the no-circling area, and a circling approach may not be possible if the no-circling area is entered before reaching the missed approach point.  To avoid this limitation it is advisable to descend to the MDA as early as possible to achieve visual contact before entering the no-circling area.
Why are the rules different for day and night?
At night it may not be possible to maintain visual clearance from obstacles even if those obstacles are lit or shown on instrument approach charts.  For this reason the rules for circling at night require that the MDA is maintained until in a position where a normal descent can be conducted, and the aerodrome Obstacle Limitation Surfaces (OLS) are intended to enable a safe approach to be conducted in those circumstances.  (Refer AIP ENR 1.5 para 1.7.2).  However the responsibility for maintaining adequate obstacle clearance still remains with the pilot and caution should be exercised.  Descent should not be commenced or continued until obstacles that may affect a safe visual approach from the MDA are identified or passed.  
Note: Aerodromes that are Certified or Registered are required under CASR Part 139 to meet standards for the survey and monitoring of Obstacle Limitation Surfaces.  Aerodromes that are not certified or registered (referred to currently as “unlicensed” and under CASR Part 173 as “other” aerodromes) are not required to meet OLS standards and pilots should ensure that they are familiar with terrain in the circling area before conducting an approach.
 
 
Missed approach during circling
If visual reference is lost during circling, a missed approach must be executed.  However, as the position at which the missed approach is initiated can be anywhere within the circling area, and the aircraft may commence the missed approach from below the MDA, the procedure designer has no means of designing a single procedure that will ensure obstacle clearance in all cases.  
In executing a missed approach from within the circling area, the assumption is made that the area above the aerodrome is generally free of hazardous obstacles and a climbing turn should be made in that direction.  Climb overhead the aerodrome should be continued to a safe height and the aircraft then tracked to establish flight on the published missed approach.  
Why is a circling MDA published at some locations when the final approach is runway aligned?
Final approach course alignment is only one of a number of criteria that need to be met for a straight-in approach.  Unless all the requirements can be satisfied, only a circling MDA is published.
　
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本文鏈接地址：非精密進近 Non-Precision Approaches Civil Aviation Advisory Publication October 2004 CAAP 178-1(1)(11)