McKay, 30, from Turnberry, blazed a trail of seven birdies against two bogeys early in the day at Royal Pines Resort in Queensland, showing signs that she could be back in the form that won her the 2003 Australian Open.
Luna, 23, the 2003 Tenerife Ladies Open Champion, also carded seven birdies and two bogeys and both players lead by a shot over Jenni Kuosa from Finland, who had a 4-under-par 68.
Five players are two shots off the pace at 3-under-par including Mianne Bagger, who now plays out of Denmark, Italian Tullia Calzavara, Australian Heidi McCulkin and three-time Tour winner Ludivine Kreutz of France, a member of the 2005 Solheim Cup team.
McKay, who set off early in the morning, and Luna, playing in the afternoon, carded five birdies each on the front nine to make the turn in 32. On their return, they both added two more birdies and two bogeys to come home in level par 35.
I was very happy with the way I played today, said Luna, who married an Italian count Fabio Orlando Del Beccuto on 6th October, but is keeping her maiden name. My secret was to stay cool and calm. Thats going to be my strategy again for tomorrow.
Luna birdied the third, fourth, fifth, eighth, ninth, 15th and 17th holes with her two mistakes at the 10th and 18th holes. At the 18th, she drove the ball into the left rough, left her second shot short, chipped and two putted to slip back from the outright lead on 6-under-par.
McKay birdied the third, fifth, sixth, eighth, ninth, 14th and 15th holes, but three-putted at the 16th and 18th to drop two shots in the last three holes.
My first birdie settled me down and I was just really very happy with it, said McKay. Ive been working hard in the off season and I felt I managed to produce the results I was looking for, which was nice.
McKay was forced to return to the LPGA Qualifying School after a difficult season playing in America last season, but received the seventh conditional card which means that she will receive some starts in America. She still has her full Ladies European Tour exemption status thanks to her Australian win in 2003.
I have my full European status which is really nice so Ill be able to go back and play in Europe for some events, she added. Marisa Baena was on a conditional card on the LPGA Tour last year and she was able to Monday qualify and win an event which changed her status. So obviously the year Marisa had is something Im trying to emulate.
McKays memories of Australia are especially strong as she experienced her only win as a professional at Terrey Hills Golf and Country Club. At Terrey Hills, she mentioned that one of the key contributing skills was drinking lemon, lime and bitters in the Angastura Bitters Tent.
Terrey Hills was one of my happiest golfing memories and the Angastura tent was there at the right time for me so it helped me on my way to victory.
McKay admitted that she gained a lot of support from her home fans following the win, but that it is something she is yet to follow up on. Obviously people expected me to do better than what I have done and they still talk about me having won it, so hopefully two years down the line I can go for it, she said.
With 44 players under par and a field of 148 there is still room for the movers and shakers in the tournament. Last year at Royal Pines, eventual winner Karrie Webb carded a 70 and was seven shots off the lead after the opening round, but came from four shots behind on the final day to win the tournament.
She has narrowed that gap this year. Webb and tournament favorite Ai Miyazato posted 71s, which has them four shots off the lead at 1-under-par. With their capacity to shoot extremely low scores, such as Ais opening 63 in the event last year, they are both well within striking distance of capturing the title.