Mahwire was forced to take to the international sidelines after video recordings were sent to the ICC.
The world cricket controlling body then allowed Zimbabwe coaches six weeks -- stage one under their regulations -- to put his action right.
A further six weeks was then put into effect, allowing the Zimbabwe coaches further time to try and bring Mahwire's action within acceptable bounds. That period has now expired.
Marsh said: "Mahwire is still working hard with Phil Simmons. We think he is coming right. It just takes a bit of time, that's all."
Former West Indies opening batsman Simmons is coaching an extended "new' Zimbabwe team squad at the Zimbabwe Cricket Union's academy in Harare, including Mahwire, as Marsh winds down his three-year tenure.
The ZCU agreed that Mahwire's action did not conform to the laws of the game, but he is expected to be able to be back playing within the next few weeks.