Mark Walker's disappointment at Group One performer Campionessa missing a start in Saturday's Gr.1 Australian Cup (2000m) at Flemington was palpable on Wednesday.

The Matamata trainer admitted he was struggling to process news that Campionessa was left stranded as the second ballot for the weight-for-age feature, having spoken to Racing Victoria officials earlier in the week and received assurances she would secure a run.

"In recent years they haven't had a full field but with the rail out 8m, they can only run 16, whereas every other year they can run 18 with the rail in. I'm just still trying to process it,” Walker said.

"The Racing Victoria guys can't believe it themselves. Just with the prizemoney and the way they scale it and with New Zealand prizemoney so low, it's meant she's missed out. 

"We just didn't expect it. Racing Victoria didn't expect it. They're gutted as well because she's an in-form horse and should be in the race."

A Gr.2 Rich Hill Mile (1600m) winner earlier this preparation, Campionessa has finished second in Group One weight-for-age races at her last two starts, chasing Australian Group One winner and subsequent Gr.1 New Zealand Derby (2400m) winner Sharp 'N' Smart in the Herbie Dyke Stakes (2000m) at Te Rapa before her last-start second to Prowess in the New Zealand Stakes (2050m) at Pukekohe.

Walker will give consideration to an alternative race for Campionessa, who arrived in Melbourne earlier on Tuesday, but for now he's hoping for the two scratchings she needs to gain a start on Saturday.

Walker's more immediate focus will be on Group One star Imperatriz as she prepares for Friday's Gr.1 William Reid Stakes (1200m) at Moonee Valley.

Imperatriz, who floated to Melbourne a week after her gallant second placing in the Gr.1 Canterbury Stakes (1200m) at Randwick, has drawn barrier seven for the sprint feature, a gate Walker was pleased with.

"That gives us good options," said Walker, who was rapt with the I Am Invincible mare's trackwork at The Valley on Monday.

"She got around the Valley no problem at all. It's hard to knock her form right-handed but I believe she's a little bit better left-handed so all's good in that respect. 

"They don't give away Group One races over there but we're hoping she's a really good chance. Her run in Sydney was good and the winner (Artorius) ran a really good race in the George Ryder last weekend so the form has stacked up well. We're happy with her."

Walker, who has racked up multiple Group One placings in Melbourne through Distinctly Secret, Darci Brahma and Princess Coup, would love for Imperatriz to claim his first elite-level win in Victoria.

"We've been a bit unlucky on a few occasions. Princess Coup was unlucky not to win a Group One in Melbourne and Distinctly Secret was unlucky so hopefully it's our turn after the gutting news with Campionessa."

Back home, Walker has a strong hand at New Plymouth, Tauranga, Riccarton and Tauherenikau as he continues to chase down Jamie Richards' New Zealand record of 160 wins in a season.

Walker, who racked up his 150th winner for the season when Quintessa won at Pukekohe on Wednesday, has Perfect Scenario and Brando as his representatives in Saturday's Gr.2 Ultimate Mazda Japan Trophy (1600m) run under set weights and penalties conditions at Tauranga.

"We're pleased with them both,” he said. “Particularly Brando is nicely placed at the weights for that race. He's just weighted out of handicaps now at his rating. Perfectly Scenario trialled well at Taupo yesterday and he's in really good form too."