"I would have loved to have stayed but I have to leave it up to someone who has more time to spare."
Mr McLachlan said he felt the board was working well and was a credit to its members and the council.
"But my passion is getting iwi back into education as well and I want to put my time into that.
"I hope the council does not have to hold a byelection, it was not my intention to cost the council more money, but logistically I could not continue there," he said.
Rural Community Board chairwoman Shirley Trumper said Mr McLachlan's skills and advocacy would be sorely missed, but his job offer was too hard to turn down.
In an agenda report, council strategy and partnerships group manager Jean-Paul Gaston said that because of Mr McLachlan's resignation councillors had to decide on "the method and listing of candidates for a byelection and the electoral system to be used for 2019 and 2022 local elections".
They will need to decide if the council adopts a first past the post or single transferable vote election system and how names would appear on voting papers - either alphabetical, randomly on each voting document or a pseudo-random option for candidate names on voting documents.
Those decisions would also hold for the next two local body elections to be held in 2019 and 2022.
However, a byelection may not be needed if only one person puts their name forward as a candidate.
Mr Gaston said a byelection would around $19,000, with nominations opening on Thursday, August 3 and closing on Thursday, August 31.
If only one candidate is nominated that person would automatically be elected and take up their role on Tuesday, September 5.
If more than one person is nominated a public notice advising the day of election and candidates' names would be made on the same day.
If the by-election goes ahead, voting papers would start to be delivered on October 5 with the final day for voting being October 27.
Also on the agenda is a further report on the council's project to turn Rotorua into the country's first official bilingual city and the adoption of the council's "Vision 2030 Refresh".
The meeting is open to the public.
Rural Community Board
Introduced at the last election, in October 2016
Made up of four elected members and one district councillor
Two board members also sit on the council's two main committees