Mr Marshall announced on Thursday, April 4 that he intends to formally hand his resignation as Northern Tablelands MP to the Speaker of the NSW Legislative Assembly on May 13.

He has served 11 years in the seat since his election to parliament in 2013, in a by-election triggered by the resignation of independent MP Richard Torbay.

He said it was time "to move on and take up new challenges".

Now the safest seat in NSW faces another by-election - the third since the seat's recreation in 1981 and first of this current term of government (2023-2027).

By-elections are held when a vacancy arises in the Legislative Assembly - such as a resignation - and are conducted much in the same way as a general election, however it is just for individual electorates that require a vacancy to be filled.

Unlike a regular election the Speaker of the Legislative Assembly issues the writ for the holding of a by-election to the NSW Electoral Commissioner, rather than the NSW Governor.

The date on which the by-election is to be conducted is one of the matters specified in the writ issued by the speaker.

The by-election will see candidates nominated by parties and as independents, and will campaign for votes.

Residents will go to the polls on the selected date to vote and elect a new candidate to represent them in parliament, until the next NSW election in early 2027.

Voting will be the same as a regular election, with the use of an optional preferential voting system.

At the 2023 NSW Election nine candidates battled Mr Marshall for the Northern Tablelands seat, but only one of those candidates, Labor' Yvonne Langenberg, attracted more than 10 per cent of first-preference vote.

Mr Marshall won with a first preference total of 71.6 per cent and two party preferred total of 83.8 per cent - the largest margin in the state.