TradeMe users have been searching for masks more than any other item on its website, followed by hand sanitiser and desks.
The company's head of marketplace Lisa Stewart said there were 250,000 searches for masks this week, and 140,000 searches for sanitiser.
Laptops and masks were the most sold item on the website.
"We've seen thousands of searches for essential items as Kiwis try to get the things they really need during alert level 4," Stewart said.
"iPhones and freezers have also been popular search terms. Kiwis are also searching for non-essential items such as PS4s, Xbox Ones and Nintendo Switches during lockdown, although those cannot be sent to buyers during alert level 4."
Stewart said they were cracking down on exploitative pricing on in-demand items.
"Our trust and safety team monitor the site really closely, and people can use the community watch button at the bottom of every listing if things don't seem right, and our team will take a look.
"If people are found to be not following the policies we've got, we're coming down pretty hard at the moment and it's likely they'll be banned from the site."
"We're aiming to restrict the prices to somewhere pretty close to the normal or reasonable retail price."
Read more about the Covid-19 coronavirus:
- See all RNZ Covid-19 news
- Your Covid-19 questions answered - from health and jobs to keeping anxiety in check
- A timeline: How the coronavirus started, spread and stalled life in New Zealand
- Covid-19 symptoms: What they are and how they make you feel
- Touching your Face: Why do we do it and how to stop
- Scientific hand-washing advice to avoid infection
- Coronavirus: A glossary of terms
- The Coronavirus Podcast