Where has frequent, significant cold fronts during summer?

I wouldn't use the word 'frequent' during the summer months, although they are certainly a feature of the southern coastline. Often these colder frontal systems only 'clip' the coasts and don't venture too far inland. Using the State of Victoria as a classic example, the northern districts including the Murray Valley can have prolonged heat without being impacted by frontal systems that pass through the southern regions.
The Great Dividing Range runs in a general North East - South West direction in Victoria, before turning almost due West and petering out in Western Victoria. Summer frontal changes generally only impact the regions south of the Ranges. The southern Victorian coastline'( including Melbourne ) has the classic '4 seasons in one day' variability and changeability in weather patterns. Some years, a 'blocking high' is established over the Tasman Sea, and in summer this will enhance hot dry weather, with winds imported south from the dry inland, and the frontal systems barely touch the extreme southern coasts. 'Blocking highs' in winter often means less rain, and clear sunny days and more frequent cold frosty nights.
Here's the latest synoptics showing some frontal systems.

Ditto for here, cold front are frequent around these parts in summer. If we're lucky they're accompanied by storms or at least frontal rain. Nowadays though, the dreaded cold change, only bringing in lame oceanic air without any precipitation, is becoming more and more frequent. 

SE Australia, mostly Southern Victoria. Southern Victoria can barely string together 2 days of heat because of the endless parade of cold fronts clipping the coast. Frequency and perceived dogshit declines as you head inland.

Pretty much nowhere in the northern hemisphere at 35-40* latitude gets summer cold fronts and cold invasions with the regularity that SE Aus does.

Compare the temps along the Med in Europe to even inland Victoria and SE Sth Aus, and it is hilariously warmer, especially for lows. Just about everywhere in the USA aside from coastal California at the same latitudes has summers an order of a magnitude warmer than SE Australia.

Eu and the US have landmasses to the north, so can maintain summer heat at 35-40N, while SE Australia has an extremely cold ocean with no stops in between. Nothing, NOTHING to obstruct the regular parade of cold fronts and southerlies year round. The only reason why SE Australia can get any spell of heat is because a high has decided to park in the Tasman sea, directing northerlies over the SE and deflecting the cold fronts. But that often doesn't last more than a week, best case scenario 2 weeks. Anything more is hideously rare. Even in a rare best case scenario, sneaky back door southerlies often infect southern Vic every few days if not more. 

Lapland. Utsjoki Kevo's highs in July 2019 ranged from 9.7C to 30.1C.

ogimet.com/cgi-bin/gsynres?lang=en&ind=02805&ndays=30&ano=2019&mes=07&day=31&hora=20&ord=REV&Send=Send

Yeah. To sum it up, wherever in the Southern Hemisphere from 35 to 55°S is eternally beaten by cold front after cold front.