Highly alkaline. Got any concrete or mortar nearby that may have leached lime into the water (coping stones, concrete pond perhaps)? Rain is usually acid (no buffering from dissolved salts - calcium carbonates etc.) but it may have washed some pollutant into the pond.
You can buy products to change the pH such as pH down from NT Labs -
LINKThe rain water is part of your problem because it contains no dissolved minerals (soft water) that help buffer the water, stopping excessive pH swings and keeping it a narrow range around neutral.
The quick fix is to do a partial water change and to dilute the pond water with tap water. First, the tap water will be nearer to neutral (though slightly alkaline as preferred by the water company) and it will contain dissolved minerals to help buffer the water. Bear in mind that tap water has chlorine or chloramine in it as a disinfectant and will need to be neutralised using a dechlorinator such as Kusuri dechlorinator
LINK or else it will harm the fish and other pond life.
You may want to buy a buffer as well such as NT Labs Pond Buffer -
LINK or Kusuri Lithaqua -
LINK . Depending on the size of your pond it can take a lot to buffer the water and get the pH stable.
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