The masterplan says the view from the carpark in Lower Hobson St is "one of Auckland's finest, but is currently reserved for vehicles".
The masterplan calls for the carpark to be redeveloped with shops, cafes and restaurants at street level and some carparking.
The area could be redeveloped as the "city window", including the removal of the Lower Hobson St flyover.
The offer of $75 million is $10 million above its valuation of $65 million.
The offer was made public in a press statement by Matthew Hooton, who runs the public relations firm Exceltium and is a right-wing political commentator.
Mr Hooton's statement said the $75 million offer could reduce the council's operating defiecit, keep Queens Wharf and Queen Elizabeth Sqaure in public ownership and make Mayor Len Brown's vision for first-world public transport come true.
The council has agreed in principle to privatise Queen Elizabeth as part of Precinct Properties $300 million redevelopment of the downtown shopping centre and a company registered in the Caymans Island wants to build two large commercial buildings on Queens Wharf.
A statement from the mayoral office said the council would need to look at the proposal in detail before making any further comment.
Mr Brown said the offer was further evidence of strong commercial interest in the downtown area, off the back of the city rail link and other development projects.
A council source said the carpark was one of the most strategically important sites on the waterfront.
Council chief executive Stephen Town said the council received the unsolicited offer for the purchase of the downtown carpark this morning and would not be making any comment on the offer until next week.
Mr Town would not say whether other parties would be invited to make offers if the council decided to sell the carpark.
