A German tourist who was last seen almost three weeks ago has been reported missing.

Photo: Supplied/New Zealand Police

Alexander Fabrice Gaitzsch, 24, was last seen in Auckland on Sunday 1 December, after arriving in New Zealand.

Police say it's thought Mr Gaitzsch may be in the Waikato river trails area.

He's described as around 170-centimetres tall, of slim build, with brown hair.