What is it?

Neither have I - but it looks beautiful!

If you google Ctenuchidae day flying moth I think it might be what you've captured here, hope this helps.

It might also be worth looking at the Wasp Moth Euchromia polymena.

Have just been on google again and there's an insect called a Splendrous Hornet Euchromia formosia that looks just like it, so disregard the other two even though they are nice to see.

Several references for Splendrous Hornet moth but they are South African - where was this one spotted and so well photographed ?

Arctiid Moth from South Africa is Splendrous Hornet Moth
On February 20, 2013

Subject: Iridescent flying insect in South Africa
Location: Port Shepstone, +-100 km South of Durban, South Africa
February 20, 2013 4:20 am
Hi there – my mom who lives on the east coast of South Africa (Port Shepstone) took this shot of a lovely iridescent insect in her garden. Is it a type of colourful moth perhaps?
Wonderful site by the way
Cat Robinson

http://www.whatsthatbug.com/wp-content/uploads/2013/02/arctiid_south_africa_cat....

Yup, it looks like the Splendrous Hornet Moth.......... thanks all

This was taken on the beach in Southern Tanzania

It's a pair of my socks - with wings! Whatever, it's a beauty!