Thousands of people have gathered outside the New South Wales Parliament as trade unions continue to pressure the State Government over its cap on public sector pay rises.

Several hundred firefighters gathered at St Mary's Cathedral before marching down Macquarie Street to Parliament at lunchtime.

They were cheered by nurses, police and other public sector workers already gathered.

Legislation to cap public sector wage increases at 2.5 per cent passed the Upper House yesterday and is expected to be finalised in the Lower House this afternoon.

The protesters were not placated by Premier Barry O'Farrell's announcement the cap will also apply to him and all other state MPs.

Mr O'Farrell is adamant the size of the crowd will not deter him.

"Whatever numbers turn up it won't be as large as the protests that took place on the 26th of March this year at the state election," he said.

"People across NSW voted for a Government to clean up the mess Labor had left to us, to make tough decisions to restore the state's finances."

The Premier's extension of the policy also includes mayors and councillors and senior public servants.

But a man who until recently was assisting the Coalition has criticised that step.

Former NSW auditor-general Bob Sendt costed the Coalition's election promises, but says a cap on pay for senior roles will make them less attractive.

"My concern with a cap is that if it does last for some period of time, or indefinitely, the remuneration for that position might become less attractive for people and reduce the field of candidates," Mr Sendt said.

Unions insist nurses and fire fighters who took part in today's protest march did not compromise front-line services.

But courts and prisons across the state have been disrupted today as jail officers walked off the job over the changes.

Goulburn Prison is in lockdown with just a skeleton staff remaining.

People held in custody and due to appear in court have had their cases delayed unless a video-link is available.