The striker, who had earlier been linked with a deadline day move to Leicester City, took his tally to 11 in time added on before the break.
A free-kick swung into Forest's box by new signing Jeff Smith was cleared only as far as Paul Thirlwell, on the edge of the area.
The midfielder lobbed the ball back over the advancing Forest defence and quick-thinking Hawley raced in to head past flat-footed keeper Paul Smith from six yards.
City's 18-game unbeaten run was ended by Cheltenham Town at Ashton Gate
After their FA Cup heroics against Middlesbrough this was a lacklustre performance by jaded City, who went off to a loud cry of "what a load of rubbish".
Victory, though, came at some cost for Cheltenham, as their new signing Scott Brown set an unwanted record, being stretchered off for the second time in this fixture this term.
When he was a City player his injury was not serious, but this time it appears much more serious.
Now, having switched clubs, his first start lasted only ten minutes before he collided with new team-mate Damian Spencer on the edge of the box.
There was a long hold-up before he could be rushed to hospital with a suspected broken leg.
The visitors, always the better side on the night, won it with a shot from Spencer just before the break.
City were under so much first-half pressure they were finding it difficult to get any shape into their game, and more than half an hour went by before Wayne Andrews got in their first shot, which drew a dive from Shane Higgs.
But it was Cheltenham's night and after 44 minutes they got the goal they deserved when Jamie McAllister gave away one of several free-kicks, just inside the left touchline.
John Melligan drove it in, Spencer's header was blocked, but came back to him and he tucked the second chance into the bottom right corner.