First-round leader Kate Golden was one shot back after a round of 74. She was joined by Namika Omata at 141, while 2002 Women's British Open winner Karrie Webb was part of a group at 2-under-par 142.
Matthew was three shots off the lead to start the round and faltered early with a two-putt bogey at the par-5 third. The Scot responded with a birdie at the next par-5 she encountered, the ninth, to make the turn at minus-2 as wind and rain plagued the Summerlea Golf & Country Club layout.
At the par-5 11th, Matthew chipped within 10 feet of the hole to set up a birdie but gave the shot back with a bogey at the par-3 13th after she failed to save par from five feet.
Matthew came right back at the 14th, another par-5, with a two-putt birdie from 25 feet. At the par-4 16th, Matthew hit a wedge to 20 feet and drained the birdie putt.
'It was a lot windier out there today,' said Matthew. 'I've been really steady this year. I haven't really had any low rounds but I've been playing well all year.'
Ellis' round was highlighted by an eagle at the par-5 third. She hit a pitching wedge with her third shot and the ball landed three yards from the pin and spun back into the hole. Ellis added three birdies but also found trouble with two bogeys and a double-bogey for a round of 71.
'That was the toughest 1-under I've shot in a long time,' said Ellis, whose tie for second at last week's Women's British Open marked the best finish of her two-year career on the LPGA Tour. 'If the wind keeps blowing tomorrow and the next day the same sort of scenario is going to happen.'
Park, who earned her second career victory on the LPGA Tour in a playoff over Hee-Won Han at the Big Apple Classic in July, played a bogey-free round of 71 for her share of first.
'Today I just had 17 pars and then one birdie on hole No. 9,' said Park. 'I made some really good up-and-downs today.'
Webb collected three birdies and one bogey for a 70 to finish two shots off the lead. The Australian, who earned her sixth major championship last week at Turnberry, was joined in sixth place by Meg Mallon (71) and Mi Hyun Kim (74).
The conditions worsened throughout the day and the 36-hole cut ballooned to 8-over-par 152 with 79 players qualifying for the weekend. Among those to miss the cut was Rachel Teske, a two-time winner in 2002.
Full-field scores from the Bank of Montreal Canadian Women's Open