More than 300 million people are living with depression, according to the latest estimates from the World Health Organisation (WHO).

The UN agency released the estimates on Thursday ahead of World Health Day.

"These new figures are a wake-up call for all countries to re-think their approaches to mental health and to treat it with the urgency that it deserves," Xinhua news agency quoted a WHO news release as saying.

With the number of people with depression increasing more than 18 per cent from 2005 to 2015, WHO is carrying out a year-long campaign, Depression: Let's Talk, the focus of April 7's World Health Day, with the aim of encouraging more people with depression to get help.

Lack of support for people with mental disorders, coupled with a fear of stigma, prevent many from accessing the treatment they need to live healthy, productive lives.

Depression is an important risk factor for suicide, which claims hundreds of thousands of lives each year, says the report.

One of the first steps is to address issues around prejudice and discrimination.

"The continuing stigma associated with mental illness was the reason why we decided to name our campaign Depression: let's talk," said Shekhar Saxena, Director of the Department of Mental Health and Substance Abuse at WHO.

"For someone living with depression, talking to a person they trust is often the first step towards treatment and recovery."

Increased investment is also needed. In many countries, there is no, or very little, support available for people with mental health disorders. Even in high-income countries, nearly 50 per cent of people with depression do not get treatment.

On average, just three per cent of government health budgets is invested in mental health, varying from less than one percent in low-income countries to five percent in high-income countries, says the report.