This is very interesting.
Just noticed a tweet from sports industry which might give an indication of a Kayo audience.
It's relating to an AFL game, but no reason why this wouldn't work this across the board.
It says the Carlton vs Western Bulldogs game got the highest rating for the round on STV of 325k (quoting some article), which included Kayo.
BUT
The actual Fox rating was publicly released as 218k, meaning we might be able to surmise that Kayo was 107k (or one third of the the total ratings).
This means each time we see a Fox rating for a game, can probably pump it up by 50% to get a truer indication of the actual eyes watching.