His first two goals put them firmly in front and his third was a 30-yard cracker as he weighed up a pass from Mitchell Cole.
It all left hapless City possibly wondering whether they could afford to give him the match ball on a day they announced losses of £1.9million.
Among the pre-season favourites for promotion they are now facing a dog fight in the relegation zone.
Southend, easy and confident in everything they did, were simply too good for them.
After three defeats in 10 days - which saw eight goals against them - City had reshuffled their defence and left Marcus Stewart, their summer signing from Sunderland, to start things on the bench.
Northern Ireland striker Jamie Quinn, on a month's loan from Peterborough, was used as a sole striker with young David Cotterill as back-up.
But it is confidence they need and this was missing as Scott Murray clearly showed when he broke down the right and then paused looking for someone to pass to when a shot was clearly on.
That home chance was down to a mistake by Adam Barrett but the centre-back was soon rescuing Southend by clearing up as Louis Carey curled a dangerous ball into the six-yard box.
While City were labouring, Southend were all sweetness and it saw them go ahead midway through the half.
With their back line falling back, City left Luke Guttridge unmarked and from 25 yards he cracked in a shot with his left foot. Keeper Steve Phillips managed to beat it away but could not hold it and Eastwood was in for the first of his goals.
The reverse stung City and Luke Wilkshire won three corners but then luck was with them when Eastwood was floored by Bradley Orr.
It looked a clear penalty but the referee decided the point of contact was just outside the box and Southend had to settle for a Kevin Maher free-kick skimming the bar.
Justice was done right at the start of the second half. Cole's corner was headed on by Shaun Goater and Eastwood was there again to provide the finish.
He the added a superb third in the 75th minute to complete a fine afternoon's work.