Scientists are scratching their heads over mass groups of mostly juvenile Great White Sharks gathering in the Kaipara Harbour.

Numbers have been growing over the past month, but it's not clear where they're coming from.

DoC's Clinton Duffy says eight sharks were seen at one spot a fortnight ago.

He says work's being done to find out if there are any breeding grounds in the area, because there's little known about the shark's habits around the North Island.