COVID-19 lockdowns are "probably the most sustained and serious stress that many of us are going to face in our lifetime", the chief psychiatrist in NSW says.

People living in lockdown should take simple steps like setting goals and asking others if they are okay to help prevent long-lasting impacts on mental health, the NSW chief psychiatrist says.

If they notice they are not sleeping well, feel irritable, or are drinking too much, they should think about seeking help from mental health professionals, he advised.

Dr Murray Wright described COVID-19 lockdowns as "probably the most sustained and serious stress that many of us are going to face in our lifetime".

He advised having a plan to monitor and manage the stress.

It is important to recreate structure in our lives, which many have lost due to stay-at-home orders, he said.

Dr Wright suggested doing daily exercise and contacting loved ones.

He said it is important to talk about the pandemic and wellbeing with others, including asking them if they are okay.

"That never does harm, it often does a world of good," Dr Wright said on Thursday.

Talking about wellbeing "will minimise the long-term impact of mental health issues long after the pandemic has gone," he said.

His other advice included setting goals to manage stress every day and reviewing them, and monitoring diet, sleep and alcohol intake.

"None of us always executes the perfect plan, but it's important to review it and renew it on a daily basis," he said.

Mental health warning signs include difficulties with sleep and concentration, undue fatigue, feeling overwhelmed and irritable, and consuming too much alcohol.

If people detect these warning signs in themselves or others, they should think about how they can change their approach or if they need help.

Dr Wright said NSW Health had invested in making telephone health services more widely available.

He also advised looking up online organisations like Beyond Blue which have specific advice for managing the pandemic.

People should cut each other some slack as everyone is experiencing stress and may not respond to situations in their usual way, he said.

The state's mental health system has itself been hit by the pandemic, after a patient at Nepean Hospital's mental health unit tested positive to COVID-19.

Health Minister Brad Hazzard acknowledged the hospital was under "substantial pressure" because its workers had been exposed to the virus and in some cases had to isolate for 14 days.

The unit's 30 or so patients have complex needs as some have "severe disassociated thoughts", Mr Hazzard said.

Positive cases have been transferred to the hospital's dedicated COVID ward.