With the holiday road toll standing at three deaths since Christmas Eve, the same as at this time last summer, an agency spokeswoman appealed for drivers to allow plenty of time for journeys "so they don't get impatient".
"When they get impatient, that's when they make bad decisions," she said.
The agency predicts heavy flows from Auckland to Northland, the Waikato and Bay of Plenty again tomorrow.
It advises holidaymakers to avoid the heaviest periods, such as from 10am to 1pm on both the Puhoi-Wellsford section of SH1 and eastbound along SH2 through Maramarua to the Coromandel Peninsula and Tauranga.
Those heading south to Hamilton and beyond should avoid peak travel from 10.30am to 12.30pm.
Traffic is likely - based on a Transport Agency's study of previous summer holiday travel patterns - to be heavy between Paeroa and Tauranga via the Karangahake Gorge for much of tomorrow, but particularly over the four hours from 11am.
Congestion is expected to continue on the main road north from Auckland every day this week, and in the opposite direction from Friday until about 6.30pm on Monday, the final public holiday of the Christmas-New Year period.
The Transport Agency is repeating its standard advice to holidaymakers - who will include those heading for the three-day Northern Bass festival near Mangawhai from Tuesday until New Year's Eve - to consider the alternative route from Auckland along SH16 via Helensville.
Westbound traffic heading back to Auckland through Maramarua after New Year's Day is likely to be heavy during most daylight hours from about 11am on Saturday until the following Tuesday evening, even after many offices reopen from the holidays.
Solid traffic flows are predicted every day for the next week on the South Island's most popular holiday run, from Cromwell to Queenstown via SH6, and in the opposite direction from New Year's Day until late on Monday.
The road toll followed a crash in which two people were killed on SH1 near Tokoroa on Boxing Day, and a three-car smash in which one person died on the highway at Oakleigh south of Whangarei on Christmas Day.
Police attended two separate crashes on SH1 about 7km south of Oakleigh early this afternoon, each involving a car in collision with a truck and only about 1km apart. Two injured occupants of a car reported to police as having crossed the highway's centre line at its intersection with Cotton Rd were taken to Whangarei Base Hospital by ambulance.
But nobody was reported hurt in the second collision, further south, between a car and a petrol tanker.
The police reported traffic flowing past both crash sites.
Melting tarmac has added to the woes of holiday-makers stewing in heavy traffic in the Waikato.
The Transport Agency has sent contractors to inspect the road surface about two kilometres south of Hampton Downs, where traffic has been reduced to a crawl in both southbound lanes.
A northbound motorist said traffic was almost at a standstill in the other direction for about 1km when she drove past the site north of Rangiriri at about 4pm, and she could smell melting tar.
"You could smell the tar because I had my windows down, and I could hear it sticking to the wheels [of southbound vehicles] as they drove past." she told the Herald.
"There were two lanes of traffic, bumper-to-bumper."
A Transport Agency spokeswoman confirmed that hot tarmac had started "bleeding" on what had already been a day of slow traffic through the area.
"It's very, very slow - it's been heavy through there all day - one of our traffic hot spots," she said.
"But we don't believe that's related to the tar seal. It's just that the traffic is going slow through there anyway, which means drivers are noticing the tar seal and they wouldn't if they were going at normal speeds."
The spokeswoman said callers to the agency's contact centre were being assured their tyres would not damaged.
Contractors had been sent to check the site, but she believed there was little they could do about the tar, and temperatures were likely to ease soon.
She said heavy congestion at the turnoff to Thames from the Waikato Expressway at the base of the Bombay Hills had eased, as had that on State Highway One north of Auckland past Puhoi.
But northbound traffic was still slow through Warkworth and the Dome Valley to Wellsford.
