Dealing with the overflow of either a pond or a tank is relatively easy, the main thing to consider is where you are going to take it. As far as the tanks go I would plumb the overflow back into the house gutter system and let it go to where the outlet is for that, now that’s either out to the street or into a pit of some kind.
As far as the pond goes I would either let it over flow at one point that is through a some nylon fly screen or some other fine mesh to make sure that nothing leaves the pond you want to keep in there…like fish for example. This of course is optional and relative to your situation, the other option is that a pipe can be set at the required level, usually done during construction but not essential, this can be put in after.
But determine the level and this pipe will then collect the overflow, keep in mind that it has to be large enough to handle the volume of water you be dealing with, the bigger the pond the bigger the pipe, trial and error will be your test… the pipe will then transport the water to its destination, if it’s a smallish pond then it won’t hurt feeding this back into the garden as it often has good nutrients in it.
This I would do using the likes of an agg line, which is laid into your garden usually about a foot deep (300mm) and this if long enough will provide some good deep watering for that garden. It’s a good idea to bring the end of this pipe to the surface though and provided you are still beneath the level of the pond it won’t back up with water, this will then just overflow onto the ground, but if your pipe is fed through many gardens then there is a very good chance that it will soak away before filling up.
Like most things to do with this aspect of your landscape plan is that it’s best to get this kind of work done first… then you will not be hindered by your plants and other structures… I know this is not always possible and is relative to your situation, but if you have the opportunity then get this kind of work planned out and done before it becomes difficult.
Hope this helps !!