Fears for man, 39, and a three-month-old baby last seen in Sydney's Royal National Park after dark on Anzac Day

A desperate search is underway to find a missing man and a three-month-old baby.  

Ben Cullen, 39, was last seen with his son Roam on Sir Bertram Stevens Drive at the Royal National Park in Sydney's south at about 8.15pm on Thursday.

A missing person's alert was issued for the pair by New South Wales Police about midnight, asking for assistance to locate them.

'Ben is described as being of Caucasian appearance, 177cm tall, medium build, with short brown hair and blue eyes.' police said.

'He was last seen wearing a navy-blue shirt and black pants.

'They are known to frequent the Kurnell, Otford and Sutherland Shire areas.'

Anyone with information into their whereabouts is urged to contact police immediately.

Ben, 39, was last seen with little Roam on Sir Bertram Stevens Drive at the Royal National Park in Sydney 's south about 8.15pm on Thursday

Ben, 39, was last seen with little Roam on Sir Bertram Stevens Drive at the Royal National Park in Sydney 's south about 8.15pm on Thursday

A missing person's alert was issued for the pair by New South Wales Police about midnight, asking for assistance to locate them

A missing person's alert was issued for the pair by New South Wales Police about midnight, asking for assistance to locate them

Anyone with information into their whereabouts is urged to contact police immediately

Anyone with information into their whereabouts is urged to contact police immediately