If it is just a steel conduit riser and is only surface rust you probably have years before it becomes a problem. Depends on how severe the rust is.
The best, and likely only fix, will be to call a couple of good electrician to give you an estimate. This is no simple DIY job.

This has nothing to do with the inside wiring of your home.

If it's rigid conduit with surface rust only, there is probably no immediate issue. If a hole is rotting through, have an electrician evaluate.

You can clean it up (use a Scotchbrite pad rather than a wire brush or steel wool, you can find them in the PAINT sectio of the home center. Then repaint with a good quality paint like rustoleum. The conduit there is primarily to protect physical damage, so as long as it's in decent mechanical shape, you should be fine.

Depending on your location it may be the POCOC problem.

In some places the POCO is responsible for everything up to the meter enclosure once it has been installed by the builder.

If you want to make it look nicer some Scotch-brite and paint will do the job.
A metal brush would also be OK if used carefully.
Avoid wire wheels on power tools, especially if any insulated wires are exposed through the hole.

or just rub some ospho on it .. converts rust to a primer like substance... then some nice zinc paint over it.

Wonderful stuff, Ospho (phosphoric acid & lanolin).. most all hardware/paint/building supply stores have it around here. Little goes a long ways. Amazed at what a totally rusted shed looks like years later after using ospho here topped with rustolium.

Here is a link that might be useful: http://www.ospho.com/

"or just rub some ospho on it .. converts rust to a primer like substance..."

Given the usual high prices for anything dealing with boats and the marine environment there are numerous rust conversion products on the market (try an auto paint store or larger auto parts store).

You still need to remove loose rust, but not surface rust that is tightly adhered to the surface.

It will make a smoother surface if you at least knock off any 'peaks' in the rust.

I would start by evaluating why it rusted in the first place.