Sydney's tight rental market has shown some marginal improvement, new figures suggest.

Data released by the REINSW shows vacancies Sydney's Middle and Outer Suburbs crept up for October, while vacancies fell slightly in the Inner Suburbs. REINSW president Wayne Stewart said, while vacancy rates overall had remained steady for two months, the city remained in "crisis".

"The rental crisis which has dramatically reduced housing affordability across NSW requires immediate and sustained action by both state and federal governments to stimulate investment in the residential property market," Stewart said.

Stewart called the rental outlook "depressed", and urged action to increase development and property investment.

"There will be no relief for tenants until we see interventionist action to encourage private investment as well as slashing planning red tape which is slowing new residential development," he said.