Luxury properties including farmland and homes are being sold to wealthy foreign investors without being available to New Zealand buyers.
Up to 100 farms, businesses and high-end homes will be sold at property expos in the UK and Switzerland by real estate firm Bayleys.
Bayleys national country manager Richard Graham said some property owners had chosen to sell directly to offshore investors before placing them on the market domestically.
Graham said: "Demand from the local market has been tough and some vendors have chosen to sell their properties overseas.
"There is this feeling that people are coming in and buying up the whole of New Zealand when it's not actually the case."
But the sell-off has provoked an outcry among Opposition politicians who say prospective Kiwi homeowners should have an equal opportunity to buy properties.
Labour associate finance spokesman David Parker said the lax laws on foreign ownership meant overseas investors could pay less tax than New Zealanders.
He said: "I absolutely think that if they are marketing first to overseas people then that is terrible. If they are selling it to overseas investors it's only because they are willing to pay more.
"Do we want to live in a country where our most successful New Zealanders can't afford to buy our best assets?"
Green Party co-leader Metiria Turei said housing was becoming less affordable to New Zealanders because of foreign investment. "It's partly because of this drive by companies like Bayleys who are touting New Zealand land to these overseas buyers without New Zealand getting a look in."
She said property ownership levels here were far lower than in countries such as Australia and Britain and were decreasing.
A TV3 poll this week found 75 per cent of New Zealanders want to see tighter controls around foreign ownership of lands. The intense public interest was sparked by the possible sale of the Crafar farms to the Chinese-owned company Natural Dairy Holdings.
A spokesman for Finance Minister Bill English said he would not comment on private sales.
English is looking at tightening up the Overseas Investment Act - and Prime Minister John Key has said he doesn't want Kiwis to become tenants in their own country.
Overseas Investment Office manager Annelies McClure said approval was only needed where "sensitive" areas of over 5ha of farmland was being sold.