Southland was smashed 84-0 by Canterbury in Christchurch last Sunday, conceding 12 converted tries. It trailed 49-0 at the break.
Mackintosh said the team felt it had also let the coaching staff down.
"If you look at how far we have come as a team in the last decade, to succumb to such a heavy loss is really disappointing. We let ourselves and just as importantly our province down and each and every one of us knows it," he said.
"One of the key attributes of being a Stags player is looking your mate in the eye after the game, knowing that you worked really hard for him. Sunday's changing room was a very sombre experience in this regard."
Mackintosh has written an open letter to fans which is scheduled to appear in The Southland Times this morning.
He admitted Southland supporters at the ground in Christchurch last Saturday might have felt a little stunned by the result.
The big prop said the past week had been one of the toughest with Southland since he started playing for the team in 2004.
Despite the heavy loss, Southland still has an outside chance of making the playoffs. He was calling for support from the team for this Sunday's game against Taranaki in Invercargill.
If things do fall Southland's way, Mackintosh is eying up a derby clash.
"Although the season hasn't gone to plan I think we have showed some resolve in the mid part of the season to set ourselves in position for the playoffs. Two late wins could see us play Otago at home in the playoffs. Wouldn't that be great?"
Championship team Tasman was a 17-3 winner over Premiership side Bay of Plenty in Nelson last night, building a commanding halftime lead and driving a dagger into the Steamers' chances of survival in the ITM Cup top flight, APNZ reports.
It was hungrier, more accurate and more creative clinical as it arrested a three-match losing streak.
Tasman 17 (Tom Marshall, Quentin MacDonald tries; Marshall 2 con, pen), Bay of Plenty 3 (Nick McCashin pen). Halftime: 17-3.
