Wellington's mayor is on a mission to improve the quality of rental properties in the capital.

Latest Trade Me figures show the number of available rentals in Wellington has plummeted, and the cost has jumped.

The figures show the number of rentals available in the capital has decreased by 70% year on year.

Justin Lester said he's not only working to get more houses built in the city but to bring the current ones up to standard with the voluntary warrant of fitness scheme.

"There's a moral obligation to make sure nobody is living in slums.

"I don't think any landlord wants to be having their tenants living in mouldy or leaky accommodation.," he said.

Mr Lester said the problem has been building for years, and he's already working towards solutions.

"I'm keen to partner with central government this year to ensure we have more consent being delivered, more houses being built, and a greater concentration of dwellings within the Wellington CBD."

Justin Lester warns it could take two years to catch up and meet the current demand for rentals.