The former BuffyThe Vampire Slayer writer/producer is writing Episodes 3, 5 and 7, as well as co-writing Episode 8 with comic book writer Ryan Scott.
Espenson wrote on Twitter: “It is almost all written, yes! All episodes but last one are in the pipeline at some stage.”
Asked if there was any news on a fifth series, she responded: “No word on next season yet.”
Following her work on Torchwood, Espenson will be co-writing the pilot episode for a US remake of Randall & Hopkirk (Deceased) for the SyFy channel. The classic British detective show aired on ITV from 1969-70 and was most recently remade by Vic Reeves and Bob Mortimer for BBC One in 2000.
Torchwood: Miracle Day will begin on BBC One in the UK and the Starz channel in the US in July.