Why Timing Isn't the Lever That Matters
Fibre's role depends on it being present in the gut consistently, day after day, not on when in a 24-hour period it's eaten. There's no published NHMRC guidance recommending a specific time of day for fibre intake, in contrast to some other dietary advice that does specify timing. Total daily amount and consistency across days are the variables actually tied to the benchmarks.
A Range of Reasonable Options
Fibre fits into breakfast, lunch, dinner, or anywhere in between, in whatever combination suits a given day. Spreading fibre intake across multiple meals rather than loading it all into one is a reasonable approach for comfort, since a very large single dose is more likely to cause temporary digestive discomfort than the same total spread out, but that's a tolerance consideration, not a claim that one specific time of day works better than another.
The Practical Takeaway
Pick whichever time or times actually fit into an existing routine, since the fibre that gets eaten consistently is more useful than the fibre that would be "optimal" at a specific hour but doesn't happen reliably. Consistency, not timing, is the lever that actually matters.