The incident happened shortly before 1pm on a farm at Mahuta, near Dargaville.
It is understood police are still at the scene and no further information is available.
The name of the victim, said to be an Auckland businessman, has yet to be released.
The man was among a group of seven on a week-long hunt organised by the Northland Hunt Club. They were near the top of a hill when the lightning struck. It is understood the victim's wife was among the party.
A policeman told Radio NZ that the victim was riding at the rear of the group when the lighting struck.
The six others were taken to nearby Dargaville and on to hospital in Whangarei, where at least four are undergoing treatment for electric shock.
The Metservice has issued a warning about the violent storm band affecting the top half of the North Island down to the drought-stricken Waikato, where significant rain is falling.
The storm conditions were expected to ease later in the afternoon.