"I've been thinking about going into consulting for a while now, most likely in terms of finance and local government."
He said he was not sure if he and his wife would stay in Rotorua and that was a decision they would make in the coming months.
He said he had a lot of leave owing which was why his resignation seemed so sudden.
Mr Foster became the council's chief operating officer in October 2014, with Thomas Colle taking over the chief financial officer role.
Council chief executive Geoff Williams told the Rotorua Daily Post Mr Foster had been planning to enter the consulting industry and "has now taken the opportunity to do so".
"Due to pressing external commitments, Dave is finishing this week," Mr Williams said.
"He brought a huge amount of local government knowledge to his roles with Rotorua Lakes Council.
"As chief financial officer he introduced a sustainable new financial framework and in recent times has led a large team consisting of operations across the organisation, overseeing major work, including the introduction of new waste management services and wastewater projects."
A replacement process had yet to be decided, Mr Williams said.