Here are the requirements for getting them a visitor's visa, and there is also a link there about sponsoring them for their expenses.
http://www.immigration.govt.nz/migra...rs/default.htm They should not need medicals for just a month's visit.
Normal holidaymakers don't usually get refused. Apart from the serious matters mentioned on that page that can mean refusal, INZ can refuse people if they don't think there is good evidence that the people have strong ties in their home country to go back to, so they might become overstayers.