An electoral law expert says a vote counting error could have been easily avoided.

The Electoral Commission is conducting a full check, after results from three polling booths were recorded incorrectly.

The data entry slip-up saw the Leighton Baker Party and the New Conservatives score votes meant for National.

Barrister Graeme Edgeler says it should have triggered alerts when National got no votes at one booth.

"You should have two independent people both inputting the data, and then the computer checks if they're the same. I would have assumed they did that, and the fact that this wasn't caught suggested they don't."

LISTEN ABOVE