The 42-year-old was in command of the tournament after chipping in for eagle at the par-5 14th to take a two shot lead then almost holing out again at the par-3 next.
But things quickly soured for the seven-time major winner as she missed a short par putt at 16 to fall into a tie for the lead with fast finishing Mi Hyang Lee, who was already in the clubhouse with a brilliant 66, then hit a submerged log at the 17th.
Webb hit what she thought was a perfect tee shot at the penultimate hole only to be told her ball had kicked into a fairway bunker.
“I bent down, picked my tee up, thought I hit a perfect drive there,” said a still shell shocked Webb after the round.
“When I stood up, I went to tell Johnny (her caddie) that I absolutely knotted my 3-wood exactly how I wanted and he said it kicked into the bunker.
“I have no idea how it did that.”
The news only got worse for Webb, however, as she was unable to advance the ball towards the green with her second and was forced to play out backwards.
Her third found a greenside bunker and, unable to get up and down in two, she took a six that saw her fall two behind with only the par-5 last hole to play.
There was more drama in store at the final hole, though, where the lack of a leaderboard saw Webb uncertain about her position in the tournament as she stood over her third shot from a greenside bunker.
“No leaderboard on the last, so I didn't know I had to make that,” said a clearly upset Webb afterwards.“It's pretty bad to not have a leaderboard on the last. Well, that's the first time I've ever been - that hasn't had a leaderboard on 18.”
She said the uncertainty affected how she played her bunker shot.
“Well, my bunker shot, I said to Johnny, ‘I don't know if I need to hole this or get it up-and-down’.
“Imagine if you went for it and overplayed it and you only had to get it up-and-down.
“I was trying to make it but also not being overly aggressive.”
She hit it close enough to make birdie, enough to share second, but will head to this week’s Women’s British Open with mixed feelings.
“Very gutted,” she said of the disappointment at what had unfolded over the closing holes.
“I was on a high, making the eagle, and then I had a very nice up-and-down on the next. “Obviously there were nerves there, but there was a good calmness there. I felt really good about how I played this week and I actually felt good about how calm I was today, considering it's been a while since I've been in contention.
“Hopefully that bodes well for next week.”
Minjee Lee was next best of the Australians finishing eighth alone at 2-under courtesy of a closing 69.
The West Australian was one of only 11 players to finish the week in red figures and will be among the favourites next week at Kingsbarns.
“I'm striking it pretty well,” she said of her play in Scotland.
“I think I probably need to assess the wind a little bit better. Hopefully next week I'm on my game a little bit more and hopefully some of my putts go in, as well.”
Su Oh also closed with 69 to move nine spots up the standings to T30 while Sarah Jane Smith dropped 10 places to T63 after a final round 74.
WEEKEND PINS - 17TH HOLE WOLLONGONG GOLF CLUB: