Gun jockey Matthew Cameron is hoping all the pieces of the puzzle that is Saturday's Gr.1 Harcourts Thorndon Mile (1600m) at Trentham might have fallen into place for him.

Cameron had initially accepted the ride on the Annabel Neasham-trained Holyfield but when that Aussie raider was withdrawn on Wednesday, the two-time champion jockey snapped up the mount on the Murray Baker and Andrew Forsman-trained The Chosen One.

It presents Cameron with a gilt-edged opportunity to win his first Group One race since the 2019 New Zealand 2000 Guineas (1600m) at Riccarton, when he rode Madison County, ironically at the expense of The Chosen One, whom he had ridden to win his first two starts.

"It worked out pretty well back then so hopefully it can work out for me on The Chosen One this time," Cameron said.

"It was a bit up in the air which horse I was riding. I was booked to ride Annabel Neasham's horse, then they pulled out at the last minute and luckily enough, Murray and Andrew hadn't panicked about getting a ride for The Chosen One and I was pleased to get back on him."

A winner of six of his 36 starts, Savabeel six-year-old The Chosen One has been Group One-placed on both sides of the Tasman and won a Gr.2 Herbert Power Stakes (2400m) in Melbourne.

He has contested the last three Gr.1 Melbourne Cups (3200m) at Flemington, finishing fourth and fifth the last two years and has placed in a Gr.1 Sydney Cup (3200m) at Randwick and a Gr.1 Caulfield Cup (2400m).

TAB bookmakers have him as a $6 third favourite for the Thorndon Mile behind Two Illicit at $3.60 and Coventina Bay at $3.80, largely because he carries such a favourable 55kg impost.

"He gets in really well at the weights, with the set weights and penalties," Cameron said.

"He hasn't won a race for a little while but he trialled really well and he was a little bit big then too. He's going to strip a lot fitter for that and with the polish Murray and Andrew have put on him leading into the race he should be ready to go, so I'm really looking forward to it.

"I rode him quite a bit as a three-year-old so I know him quite well. I've ridden him at the track and in trials and I'm looking forward to getting back on him in a big race."

Cameron, who is just four wins away from 100 black-type victories, has eight rides at Trentham, including Gr.3 NZ Campus Of Innovation & Sport Wellington Cup (3200m) favourite Uareastar and Sassy Merlot in the Gr.3 New Zealand Bloodstock Desert Gold Stakes (1600m).

"I like Sassy Merlot. She ran a good solid race last time and only just got beaten the last bit. She steps up to the mile but the way she jumps and sets herself on the pace to do her own thing, she's got to be a great chance in a pretty even race," Cameron said.

"I managed to pick up Uareastar in the Cup with Lisa Allpress out injured. The horse is going really well. The two miles is always a query the first time but Fraser (Auret, trainer) has his horses fit and I'm looking forward to riding her."

Cameron, who also takes three rides at Ellerslie on Sunday, said he was really enjoying his riding at present, having taken a hiatus from riding at Christmas last year before returning to the saddle earlier this season.

"I just haven't been able to win a big one yet but I've been close and I feel I'm riding well," he said.

"I haven't stuffed up too many anyway, which is always good, and I just need the one big race and that will really set me off again. I missed these summer carnivals last year so it's good to be back out there and I'm happy anyway."