Conditions at Eyes Meadow deteriorated as the afternoon progressed, with steady rain which began midway through the first half and never relented.
The weather led to a scrappy first half which only sprung to life in the final ten minutes.
On 36 minutes a great pass from Freya Wilson picked out Alice Faulding, her shot was saved and only half cleared before Mia Carnelley saw her shot go just over the bar.
Two minutes before half-time, a long ball down the right set Abi Bancroft free and she brought a good save out of Serene Linnington in the Dynamo goal.
Belper broke the deadlock in the 44th minute when Wilson was quickest to react to a looping ball inside the penalty area and lofted her header into the net.
A Sammy Bennett flick-on in the 51st minute found Faulding but the shot was just wide. However seconds later the Nailers doubled their lead when a Wilson through-ball found Faulding who produced a trademark cool finish.
The tie was effectively secured on 58 minutes when Carnelley fired in a great goal from range on the right hand side.
In the 64th minute Dynamo were awarded a penalty following a foul and Caitlyn Else scored it to reduce the arrears.
However within a minute Belper restored their three goal lead when Faulding finished nicely running in from the right.
Faulding was on the scoresheet again in the 78th minute to complete her hat-trick when Anna Macluskey rolled the ball into her path and she slotted home.
Belper showed their class going forward late on and with seven minutes remaining Faulding laid the ball off for Alex Watson who produced a superb finish to make it six.
The final goal came in the 84th minute when Belper gained possession almost straight from the kick-off and Faulding played a one-two with Carnelley before finishing inside the six-yard box.
The full-time whistle brought down the curtain on Dom Anderson's reign as manager, he announced he was stepping down after the match and the club would like to place on record their thanks to him for the job he has done.
It was also the final appearance in the yellow & black for Freya Wilson who is moving to Sheffield FC and we wish her the best of luck as she steps up the pyramid.
Elsewhere in the Cup, there were victories for Chesterfield Ladies First Team, Chesterfield Ladies U23s, Long Eaton United, Borrowash Victoria, Wirksworth Colts, Draycott Victoria and Derby County U21s. Only two home teams prevailed out of the eight ties played.
The Reserves, with a depleted squad were defeated 4-1 at Alton Manor by Gresley Rovers, with Olivia Froggatt on the scoresheet for Belper.
Duffield Dynamo (squad): Macpherson (C), Linnington, Chambers, Dobson, Hill, Potts, Wakefield, Speck, Handford, Else, Sheldon, Horsman, Spencer.
Belper Town: Varnam, Longdon, Claxton, Cowen, Pickard, Carnelley, Watson, White, Wilson (C), Faulding, Bancroft.
Subs: Buxton, Macluskey, Bennett
Referee: Chris Petcu