There's a whiff of dirty politics on the campaign trail, with advertisements coming up to election day getting personal.

National has released an attack ad called "Let's Tax This".

WATCH NATIONAL'S FULL AD ABOVE

Meanwhile, Gareth Morgan's Opportunities Party is urging voters to "Stop Peters" and a "Government from the past".

Political marketing expert Dr Edward Elder told Larry Williams it isn't unusual - even in New Zealand where things are more cordial.

"While they're attack ads, there is almost a sort of blunting of the instrument with the kind of silliness of it."

LISTEN ABOVE AS DR EDWARD ELDER SPEAKS WITH LARRY WILLIAMS