The striker, 22, spent six years in England at Sunderland, Sheffield Wednesday and Hull City without ever making a senior appearance - but has impressed coach Tony Walmsley during five weeks of training and trials in Gosford.
He has played alongside the likes of Bundesliga-based Mathew Leckie and Kerem Bulut for Australia's Under-20 and Under-23 teams and has been back in Australia to complete his rehabilitation from a long-term shoulder injury.
Walmsley has been so taken with the attributes of the strong and powerful frontman - that he is seeking to tie him down to a deal beyond the remainer of the season, with negotiations close to being completed.
The Mariners have played a key role in Fletcher's recovery from his shoulder reconstruction after he returned home needing to shed some kilos and regain match condition.
With Irish striker striker Roy O'Donovan serving an eight week ban for his head butt on Wellington's Manny Muscat, Fletcher's impending signing he could prove timely.
The club is also close to extending the contract of 21 year-old midfielder Josh Bingham, who scored on his starting debut in the thrilling 3-3 draw with Melbourne Victory in Geelong.
"We are getting close there," said Walmsley, whose side travel to face Adelaide United on Saturday. "He has agreed terms, we are just waiting to sign that one off."
The agreement is a one year extension, with the option of a further 12 months beyond that.
With defender Jacob Poscoliero out, there could be a starting debut for Tommy Uskok against the Reds, unless midfielder Jake McGing is instead deployed into the back four.