I noticed this afternoon that the water level in my pond had fallen
dramatically.

It's a 1000litre pond but it must have been down to at least 1/3 of
it's depth.

I did notice recently that it was lower than usual but we have had
nothing but rain recently.

The 9 fish (3 goldfish and 6 orfes) are doin ok, I've topped it up and
turned off the pump and filter to see how things go.

You've started with best suggestion, top off and turn off.

Is the filter separate from the pond, so you can tell if the leak might be
there?

As JB mentioned, I've always found the majority of my water losses due to
streams/waterfalls. This winter though, on slow flow, and only one small
waterfall in operation, I was losing more than I'd ever noticed in summer
with full flow and hot weather. I turned everything off, and isolated the
filter from the pond. Neither pond lost water, but the ~ 140 gallons of
filter (in winter mode) had lost over an inch in an hour!

In my case I turned everything back on, and considered it my easy way of
doing a water change for winter. We figure bulkheads need to be re-silicon
as soon as weather & time permit. DYI filter is in it's eleventh year.
~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us