In reply to a customer question regarding control of Gravel Groundsel
 
Gravel groundsel (Senecio skirrhodon) is, according to Massey University & other authorities, controlled by nearly all herbicides, except for a few specialist cropping herbicides.
 
But there aren't any herbicides specifically registered for gravel groundsel, so we've got to make a few informed assumptions in selecting the best product to use.  There are several products registered for the closely-related 'regular' groundsel (Senecio vulgaris) so that's a very good guide!
 
You've had success with GrassMate , but GrassMate  is going to damage your clover.  So it's likely, though not certain, that Triclo  (which contains the same major active ingredient as GrassMate) will also be effective.  Triclo  doesn't damage ryegrass or clover.  
 
2,4-D  is effective on regular groundsel, especially when the groundsel is still at a young growth stage.  But 2,4-D does some clover damage, although it can be minimised by grazing paddocks hard to get as much clover leaf as possible off before spraying.
 
I also think our product Decision could well be effective.  It's a water soluble granule, and will control many weed species of similar style to groundsels.  Decision is very popular with dairy farmers because it's both grass & clover friendly at any time of the year.  It's primarily aimed at buttercups and docks, but will also control a lot of other broadleaf pasture weeds.  I think gravel groundsel would likely be among them, though as it's not registered for groundsel I can't guarantee the result.
 
My advice would be to first try some Decision, because it's not terribly expensive, easy & safe to use, and won't damage your pasture.  
 
A 200g pack would cover about 4Ha at what I think should be about the right rate, of 50g/Ha.
 
Now (November) would be a good time to spray; anytime the groundsel is growing actively would be fine.  And it's nearly always a good plan to add a little wetter/penetrant at 100ml/100L water, for faster penetration & reduced rain risk period.  The less expensive SprayWetter  is fine for this.
 
I'm sorry I can't offer an unconditional answer; it's because nothing's been specifically registered for gravel groundsel.  But I'm about 90% sure that Decision  would be effective.  
 
If you do give it a try, let me know the results please, so I'll know more next time someone asks about gravel groundsel.