The temperature is forecast to stay in the 40s for Friday's crucial 128.2km fourth stage from suburban Norwood to Uraidla.
Porte is where he wants to be heading into the business end of the Adelaide tour, in 27th place overall and 20 seconds behind leader Caleb Ewan (Mitchelton-Scott).
The Australian has looked untroubled so far in his first stage race since the horrible crash last July that ended his Tour de France.
He wants to become the first Tour Down Under champion to defend his title.
Porte and his BMC teammates went straight to the beach at Victor Harbor to cool off at the end of Thursday's third stage.
The temperature reached the mid-40s and the stage was shortened because of the extreme weather.
Porte said on Twitter that the dip in the ocean was "the best thing about our trip to Victor Harbor".
In its 20th year, the Tour is finishing at Uraidla for the first time.
The closing kilometres of the stage features the Norton Summit climb, a staple of Adelaide cycling.
At 5.6km and an average gradient of five per cent, Norton Summit is not a daunting ascent for riders such as Porte.
But the Tour contenders will push the pace on the climb and that, combined with the brutal conditions, means a shake-up of the overall standings is certain.
Ewan will lose the overall lead he has held for two days.
After winning stage two, the 23-year-old star bungled his sprint at Victor Harbor and finished third behind Elia Viviani (Quick-Step Floors).
Porte's main rivals for the overall title are compatriot Jay McCarthy, who is fourth overall at 15 seconds, and his star Bora-Hansgrohe teammate Peter Sagan.
The three-time world road champion is fifth, one second behind McCarthy.
Australian Nathan Haas (Katusha-Alpecin) is another rider to watch and is sixth overall, 17 seconds from the lead.
The heat has forced event organisers to start the stage one hour earlier, at 10:30am, and cancel the community ride that had been scheduled to start earlier in the morning from Norwood.