The country's largest house sales website showed Auckland's asking price hit a new all-time record of $732,240 last month.
Paul McKenzie, Realestate.co.nz marketing manager, said there had been a big jump from April's average asking price of $685,426.
Mr McKenzie said a lack of listings meant sellers knew they could achieve big prices because buyers had so little choice.
"The high asking prices in Auckland, in particular, suggest that home sellers are confident they will get their price. A factor in this may be the low overall supply of homes on the market. Auckland home sellers want more for their homes than ever before," he said.