A senior Liberal and former leader Iain Evans has announced he will quit the South Australian Parliament within the next year, which will force a by-election.

Mr Evans, 55, has been the Opposition's treasury spokesman and says he will not seek a shadow cabinet role in a coming reshuffle by Liberal leader Steven Marshall.

The Liberals lost narrowly at the state election in March, getting more of the vote than Labor but winning fewer seats.

Since the poll, a former Liberal leader Martin Hamilton-Smith has quit the party's ranks to take up a ministerial role with the Labor Government.

The news of Mr Evans's plan to retire means there will be a by-election for his Adelaide foothills electorate of Davenport.

Labor says the Liberal Party should be very wary of taking Davenport for granted as the Greens poll strongly there.

Mr Evans has been the Liberal member in the seat for 21 years, served as a minister in the Olsen Liberal government and led the South Australian Liberal Party after its election loss in 2006.

There are indications several senior Liberals might announce their retirements in coming months, but leader Steven Marshall has talked down the prospect of any more colleagues making plans to go.

"I have advised state Liberal leader Steven Marshall of my intention to resign from the South Australian Parliament in the next 12 months, Mr Evans said.

"After 21 years representing the electorate of Davenport and my 46-year involvement in politics, I have made the decision to leave the SA Parliament in the next 12 months and seek a new career in the private sector."

Iain Evans followed in father's political footsteps

Liberal Iain Evans announces his departure plan from politics, flanked by two of his children and wife Fiona. ( ABC News: Nick Harmsen )

The senior Liberal, whose father Stan was a SA politician before him, says he is proud of his record.

"Having served as a Cabinet minister for five years, state Liberal leader and deputy Liberal leader, I am proud of my achievements in the South Australian Parliament," he said.

"I'd like to thank my constituents in Davenport for allowing me the honour to serve them for 21 years."

Mr Evans has spoken of the family sacrifice needed to be in politics.

"When I entered Parliament in 1993, I saw it as the highest form of community service," he said.

"Politics is a partnership and families make a big sacrifice. I'd like to thank my wife Fiona, and children Staten, Alexander, Fraser and Allison for their love and support.

"I'd also like to thank my father Stan and mother Barb, as well as my siblings for their ongoing support over my Parliamentary journey."