The Red Roses ran in four tries to secure a bonus point in the first half and another 10 after the break in a statement Pool C win over the Fijiana, who tired after a promising start to their World Cup debut.
"They rattled us a bit in their physicality and their intent to run the ball in the first half," said England captain Sarah Hunter.
"We were a bit more composed in the second half, managed to go back to being England and turned the game around."
Hooker Amy Cokayne, lock Abbie Ward and MacDonald scored tries which all had their origins in England's trademark lineout drive in the opening 27 minutes before Fiji hit back with a brilliant try.
Fiji fullback Roela Radiniyavuni cut through the England defensive line before finding Alowesi Nakoci with a lofted pass that allowed the winger a free run to the line.
England responded in kind through centre Helena Rowland after a break from flanker Sadia Kabeya but Fiji had the last word in the half through a fine individual score from centre Sesenieli Donu which cut the deficit to 24-14 at the break.
The second half was pretty much one-way traffic as Cokayne grabbed a second score, winger Lydia Thompson scored a double and MacDonald added three more tries to her tally.
Lock Zoe Aldcroft, replacement winger Abby Dow, reserve hooker Connie Powell and scrumhalf Leanne Infante all also crossed as the hapless Fijians chased shadows.
Fiji again had the last word, however, with scrumhalf Lavena Cavuru scoring a consolation try just before the final hooter.
"Fiji certainly came to the party in the first half and made it really difficult for us, which we expected," head coach Simon Middleton told reporters after the win. "We probably didn't quite handle it as well as we we'd have hoped to but that's alright, it shows that we are very human.
"We settled down at half-time and I think we showed great composure second half and got back to what we do well...
"Fiji really rattled us in the first half, they were very physical. We looked really nervous and that's understandable because this is the biggest stage and what a place to play. So really pleased with how we got composed and executed the game plan."
"I'm so proud of these girls, we were playing a professional outfit and they really got stuck in in the first half," said Fiji coach Senirusi Seruvakula.
France won the opening Pool C match 40-5 over South Africa earlier on Saturday, while hosts and reigning champions New Zealand will later complete the Eden Park triple-header in their clash against Australia.
