He told The Front Page it is important to understand how such property ownership might impact MPs' decision-making.

"It's something Parliament recognises as being important by making MPs declare what they have, but they don't actually ask them to declare what it's worth.

"So we decided to take it that step further and find out," he said.

The National Party MP for Rangitīkei, Suze Redmayne, tops the list with a property portfolio worth about $24m, most of which is a large farm valued at about $18m.

"Interestingly enough, she took office after Ian McKelvie retired, and he was the previous top.

"So, obviously, farms in that area are clearly worth quite a bit of money," Knox said.

Luxon had the most significant residential property assets.