Public servants must be warned against inappropriate political activities in the lead-up to the election, the Coalition says, hitting back at Treasury costings of the opposition’s planned tax deductions for corporate lunches.

Liberal spokeswoman Jane Hume wrote to Public Service Commissioner Gordon de Brouwer on Tuesday, angry over Treasurer Jim Chalmers’ request of department officials to cost a plan for exempting small businesses from fringe benefits tax for $20,000 of meal and entertainment expenses.