The 2020 National Rally will be in Wanganui (also known as Whanganui) from Friday 14 February to Sunday 16 February 2020. It is the 3rd weekend of February and is the weekend following the Burt Munro event.
The rally will be held at the Lakelands Holiday Park at Lake Wiritoa, just a few km South of Wanganui. See map here.
There is more information about Lake Wiritoa here, on the Visit Whanganui website.
The Lakelands Holiday Park was formerly known as Scoutlands until it was recently acquired by motorcyclists Jackie and Darryl Dowman. There is an NZ Herald article about the new owners here.