Prepare for a hot afternoon for the fourth day in a row. A top of 40 degrees is forecast today, which would see Canberra experience its longest heatwave on record.

The weather has certainly kept us on our toes this week. Last night Canberra received almost 7mm of rain in half an hour when a thunderstorm swept through. The epic lightning show lasted well into the night.

Here are today's local headlines:

"Turn those gadgets off!" That's the appeal from the authorities today as record-breaking temperatures threaten to push ACT and NSW power networks to their limits.

With temperatures expected to reach 40 degrees across most of the ACT for the fourth day in a row, the Australian Energy Market Operator has told the ACT government that very high electricity demand could cause supply shortfalls.

Cassandra Morgan reports on how the heat is taking its toll.

And it's not just power supply that's affected. Canberra Racing stands to lose $1 million in turnover after they put their horses' well-being ahead of the bottom line and moved today's meet earlier to beat the heat.

The financial cost could be higher if the event at Thoroughbred Park is affected further. Read David Polkinghorne's assessment.

The day after the death of her five-year-old daughter, Annabelle, Kathie Potts received a letter that caused her heart to break even further.

The letter, dated the day of her daughter's death, was from federal health minister Greg Hunt, announcing the government had allocated more than $500,000 to research into treating Diffuse Intrinsic Pontine Glioma (DIPG), an aggressive type of childhood brain cancer.

"I am absolutely gutted that it won’t help Annabelle," Kathie said. "But as Annabelle got joy from doing things for people, I am happy that this will help other families and stop them having to fight as hard as we did."

While the letter from Mr Hunt was a major blow to a family already in despair, the Potts family has received a huge outpouring of love from the Canberra community and the nation since the announcement of Annabelle's death earlier this week.

Bree Element has the full story.

We're about to see some more light rail vehicles rolling through the city.

The consortium behind Canberra's light rail project will start testing the vehicles night and day along the route from Gungahlin to the city from next Tuesday.

Testing trips are likely to start at low speeds along the Mitchell to city part of the route, similar to those tests already completed along the Gungahlin to Mitchell corridor, though will eventually ramp up to the 70km/h speeds expected to carry passengers during operations.

Daniel Burdon has the full details.

A storm in a water bottle is riling the residents of Yass. A treatment plant failure over Christmas saw brown water come out of taps, laced with dirt, for about three weeks as the local council assured residents it was safe to drink.

But it's all been really stirred up now by a photo of Yass Valley Council mayor Rowena Abbey drinking bottled water.

Finbar O'Mallon samples the dispute.

The worry was extreme heat could postpone Canberra United's match against Melbourne Victory but it was lightning that spoiled the party at Seiffert Oval on Thursday.

Players were ordered off the field in the 17th minute as thunder and lightning rattled through Queanbeyan and after an hour delay the game was abandoned.

Eamonn Teirnan has the story here.

The rain also interrupted Canberra Cavalry’s match against the Perth Heat at Narrabundah Ballpark on Thursday night was postponed in the top of the third due to rain.

Perth was ahead 2-0 when play was abandoned and the game will be made up on Friday with a doubleheader.