You may find the following articles of use.
http://www.casa.gov.au/wcmswr/_asset.../ops/178-1.pdf
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. Does that mean that a straight-in approach cannot be made in those cases?
No. It means that the instrument approach procedure and/or the runway approach surfaces do not meet all the associated safety standards. If on becoming visual the pilot assesses that the aircraft is in a suitable position to land straight-in, and the pilot is considers that it is safe to do so, a straight-in approach can be conducted provided the rules for circling are followed.
Those rules include the requirement to be established within the circling area before leaving the MDA. (AIP ENR 1.5 Section 1.7). If the decision is made to “circle” straight-in, the pilot should take into account that the runway and the runway approach area may not meet the standards for a straight-in approach and caution should be exercised.
www.terps.com/ifrr/nov97.pdf
“Straight-in Minimums:
Are shown on the IAP when the final
approach course is within 30 degrees
of the runway alignment and a
normal descent can be made from the
IFR altitude shown on the IAP to the
runway surface. When either the normal
rate of descent or the runway
alignment factor of 30 degrees is
exceeded, a straight-in minimum is
not published and a circling minimum
applies. The fact that a straight-in
minimum is not published does not
preclude pilots from landing straight
in if they have the active runway in
sight and have sufficient time to make
a normal approach for landing.
I hope the above helps.