It was a dramatic farewell for Sydney human rights lawyer Seth Richardson that his family says would have appealed to his wicked sense of humour.

As his family prepared for the funeral of the 52-year-old, also a regular contributor to letters to the editor in the Herald, in the Blue Mountains on Thursday a man jumped in the hearse and stole it – with Mr Richardson still in a coffin in the back.

It was an odd farewell for Seth Richardson on Thursday.

"One of the funeral guys who works for the funeral home went out to the hearse to grab the trolley to put it under the coffin and in a split second this guy jumped out of the bushes, jumped straight into the hearse and started it up," Mr Richardson's sister-in-law Hayley West said.

"The funeral guy was banging on the window going 'what the hell are you doing? You can't drive away in the hearse'.