From what ive read Spitfires did quite well defending Darwin considering the circumstances, they had a massive area to cover without the aid of a radar network, outnumbered flying 2nd hand mkvs with volks filters. Most losses were due to mechanical difficulties.
I don't think you've read accurate accounts then. The Spitfires did terribly in fighter-fighter exchange, and didn't actually shoot down many escorted Japanese bombers either, see the specific losses on each side listed in posts above on this thread. Many Spit losses were mechanical but known operational losses are excluded from the losses given above. Some unknown cause losses on missions v Zeroes could have been operational, some of the handful of Zeroes and single Type 1 (Ki-43, Oscar) lost in the whole campaign might have been operational too. But no plausible unbiased accounting of loss causes would make the fighter exchange ratio less than several to one against the Spitfires (if it 'on principle' rejects one side's loss accounts while accepting the other's it's non unbiased

).
Each side's claims far exceeded the others recorded losses, which might be why you have the misimpression you do. Perhaps your un-named source assumed Spit claims =Japanese losses without knowledge of the Japanese accounts, that results in a very inaccurate picture in this case. Both side's victory credits exceeded the other side's account of their losses by factor of 3+ , but the factor was higher wrt to Spit credits against Japanese fighters specifically.
The major escorted raids which resulted in fighter combats were mainly against the Darwin region, and one main Japanese base at Timor was launching all those raids. The Spits had radar warning, as USAAF P-40's defending Darwin the previous year had also had, from March 1942.
The numbers of escorts and Spit's which scrambled for each escorted raid combat are as follows (from Price "Spitfire V Aces"):
March 2 1943: 21 Zeroes, 24 Spits
March 15: 26 Zeroes, 27 Spits
May 2: 26 Zeroes, 33 Spits
May 9: 9 Zeroes, 5 Spits (Spits intercept strafing Zeroes)
May 28: 7 Zeroes, 6 Spits
June 20: 22 Type 1 (Oscar), 46 Spits
June 28: 27 Zeroes, 42 Spits
June 30: 27 Zeroes, 38 Spits
July 6: 25 Zeroes, 36 Spits
Besides the invalid explanations covered above, sure, later model Spitfires would probably have done better, perhaps much better depending on model, but the Spit V model was the closest contempoary of the A6M2, which was the type that particular Japanese unit, 202nd Air Group, was still using in those raids; and apparently mostly the same planes they'd used in the previous year's campaign (as 3rd Air Group). The 202nd was based in a remote place (Timor) at the end of a long supply line, and obviously flying in the same climate, so those factors can't simply be treated as 'errors' or as if they only affected one side. If one side was affected more, an unbiased analysis must consider that at least partly a demerit to that side's aircraft.
Joe