Multi-time former champion Birch was in scintillating form at the weekend and there were no surprises in his stretching his unbeaten run, but what may have come as something of a surprise was where Birch's main challenges came from, particularly at round four, near Christchurch, on Saturday.
Christchurch rider Hamish McDonald ended the day second overall on the steep course on Banks Peninsular, finishing just 49 seconds behind Birch, with Groombridge finishing third overall.
Howick's Liam Draper finished fourth overall on Saturday, and maintained his position at overall series runner-up.
Round five on Easter Monday was staged just down the road in the Berwick Forest at Mosgiel, inland from Dunedin, and again Birch was supreme, albeit winning the day by just five seconds from fellow Auckland rider Chris Power, in his tightest winning margin this season.
Draper, riding a Husqvarna TE125, finished third on Monday and remains overall runner-up for the series, although still 29 points adrift of Birch.
Draper is, however, dominating the battle-within-a-battle for under-200cc two-stroke class honours.
Groombridge finished sixth overall on Monday, just behind Wellington's multi-time national moto trials champion Jake Whitaker, but because these two South Island events were Groombridge's only enduro outings of the season thus far, he really has no chance of clinching top honours in either the overall or individual bike classifications.
The series continues with round six is at Waimiha on Saturday, May 9, and then wraps up at the seventh and final round near Tokoroa on Sunday, May 10.
