One of the problems with this subject is that most regulations are written in the language of visibility – where visibility is defined in the context of an object (from ICAO Annex 2):
Visibility. Visibility for aeronautical purposes is the greater of:
(a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background;
(b) the greatest distance at which lights in the vicinity of 1 000 candelas can be seen and identified against an unlit background.
When the subject is more associated with flight in a 'usable cue environment' (or more correctly, 'in degraded visual conditions') – a far more complex issue.
Although the certification code contains a hook for addressing such issues:
27/29.141 The rotorcraft must--
(a) Except as specifically required in the applicable section meet the flight characteristics requirements of this subpart
…
(c) Have any additional characteristic required for night or instrument operation, if certification for those kinds of operation is requested. Requirements for helicopter instrument flight are contained in Appendix B of this Part.
it is unusual for any additional characteristics to be specified for ‘night flight’ - when it relies upon the visual cue environment for maintenance of stability/control.
For that reason, CASA might be well advised to refer to CAA Paper 2007/03 ‘Helicopter Flight in Degraded Visual Conditions’ - an extremely good treatise, dealing with this very subject, and which contains appropriate recommendations.
Jim