Ideal....Subjective depending on experience. Up to 2 metres is fine IMO to run a packraft anywhere on the river. I wouldn't be keen above that in the upper sections if you are just after a paddle and not much experience.
You can run it higher of course if you know your gear and the have the ability etc. Sure, high volume flattens some rapids but it doesn't necessarily make them easier to run, or any less dangerous. It can get pretty full on in the gorge.
I know people have just made it out within an hour or so before it went mental as it comes up higher and harder than people expect on occasion. Extremely lucky.
I also have friends that have been choppered out because they grossly underestimated the time it can take in low water. Start small, watch the gauges in the lead up to rain events and become familiar with where rain is actually falling on the radar, how long and how much in the catchment.
I've paddled the lower reaches from Upper Colo bridge down at around 7.5 metres and it moves fast, its wide and lots of trees and debri in the water. I would not like to be in the gorge anywhere above this. The flat pools might be easy but the rapids require experience.
https://vimeo.com/17476713