Following the crash at 4.10pm on Friday 14 September 2018, Dorset Police was informed by the ambulance service about the incident in Hessary Street.
The collision involved a yellow Hyosung motorcycle and a white Iveco van.
The air ambulance service attended and a 25-year-old local man was taken to Southampton hospital with serious injuries. Sadly he died from his injuries on Sunday morning.
Police Sergeant Craig Tatton, of the traffic unit, said: “I am appealing for any witnesses to the collision who have not spoken with officers yet to come forward.
“I would also like to hear from anyone who saw the manner of driving of either vehicle before the incident or who has relevant dashcam footage.
Anyone with information is asked to contact Dorset Police at www.dorset.police.uk, via email at firstname.lastname@example.org or by calling 101, quoting incident number 14:333. Alternatively, contact Crimestoppers anonymously on 0800 555 111 or via www.crimestoppers-uk.org