It is understood Government officials, along with security staff, visited several high-end accommodation providers this month.
The secret visit has fuelled speculation Prince William, his bride-to-be, Kate Middleton, and his brother, Prince Harry, will visit the city, which will host three England pool games.
Monarchy New Zealand chairman Simon O'Connor said, while details of any royal visit for the Rugby World Cup were yet to be confirmed, there was speculation the trio would visit.
• Tests to stretch accomodation
"We would be keen to see them."
The royal couple will honeymoon in Kenya after their April 29 wedding, and will make their first official overseas trip as a married couple to Canada in July.
Mr O'Connor said any visit to New Zealand was likely to attract intense media interest from all around the world.
Calls to the Rugby World Cup Guest of Government Task Force, which was established to co-ordinate arrangements for the cup's Guest of Government programme, were not returned yesterday.
Larnach Castle marketing and public relations manager Deborah Price declined to comment whether officials visited its newly-opened luxury lodge, Camp Estate.
However, she confirmed bookings for the country retreat were being taking for the tournament, but some weekends were being "held".
Billionaire entrepreneur Sir Richard Branson has also been tipped as a visitor to New Zealand for the tournament.
